Chert grain size

Sedimentary rock - Silica, Clastic, Chemical: Those siliceous rocks composed of an exceptionally high amount of crystalline siliceous material, mainly the mineral quartz (especially microcrystalline quartz and fibrous chalcedony) and amorphous opal, are most commonly known as chert. A wide variety of rock names are applied to cherty rocks reflecting their colour (flint is dark chert; jasper is ...

Science. Earth Sciences. Earth Sciences questions and answers. DQuestion 1 3 pts Match the physical properties to the appropriate rock name Breccia IChoose ] I Choose ] GRAIN SIZE: sand to pebble sized; SORTING: poorly sorted; ROUNDNESS: rounded to subrounded; LAYERED: no; FOSSILS: no; ACID REACTION: no GRAIN SIZE: sand to pebble sized: SORTING ...Choose one: A. A well-sorted clastic rock is made up of different-sized particles. B. Conglomerates consist of a single grain size. C. An immature clastic rock has well-sorted, resistant grains. D. Pieces of broken rock produced by physical weathering are collectively called clasts or detritus. D. Pieces of broken rock produced by physical ...Abstract A submillimeter‐scale variation of δ18O in quartz was identified in chert and Fe‐oxide mesobands of the Brockman Iron Formation, Western Australia, using an in situ CO2‐laser fluorination ...Sandstones are sedimentary rocks that can be made up of any sand-size particles. In Mississippi, most sandstones are made up of quartz grains. Sandstones can be cemented to various degrees of competence by a few different methods. Types of cement such as opal, chert, chalcedony, and quartz typically form the strongest bonds between the …(chert) Deep seafloor where plankton with microscopic SiO 2 shells rain down. Detrital sedimentary rock (clasts are mostly rock and/or mineral fragments Grain size Texture and composition Name Depositional environment Gravel Rounded fragments; poorly sorted Conglomerate Beach headlands, natural levees, tops of alluvial fans.Study with Quizlet and memorize flashcards containing terms like Texture: Clastic (course) Composition: Chert Grain size: sand/silt-gravel (mostly gravel) sorting: poorly sorted roundness: angular Fossils: N/A Identifying markers: scratches glass, large angular chunks, Texture: Clastic (course) Composition: Quartz (Iron oxide?) Grain size: sand-pebble (mostly pebble) sorting: poorly sorted ...This type of environment is the main reason why phosphorites are commonly associated with silica and chert. Estuaries are also known as a phosphorus “trap”. ... Most phosphorite grains are sand size although particles greater than 2 mm may be present. These larger grains, referred to as nodules, can range up to several tens of centimeters ...Gypsum deposits can vary in terms of purity, crystal size, and mineral associations. The quality and characteristics of gypsum in a particular location depend on the geological history and environmental conditions of that region. Gypsum is a valuable mineral resource used in construction, agriculture, industry, and various other applications.

Udden-Wentworth Grain Scale (Sand scale) (Derived from Nichols) [1] Sand grain size has a wide range of scale, from very coarse sand until very find sand. For the grain size measurement, its use a formula proposed by Krumbein in 1934, is based on the following relationship : Where Ф (phi) size and d is the grain diameter in milimeters.Clastic sedimentary rocks are subdivided on the basis of grain size. ... chert (Pellant p. 246) chalk (Pellant p. 237) diatomite coal (Pellant p. 244). PART 1 ...Chert is a sedimentary rock composed mostly of the mineral chalcedony—cryptocrystalline silica in crystals of submicroscopic size. This type of sedimentary rock can form in parts of the deep sea where the tiny shells of siliceous organisms are concentrated, or elsewhere where underground fluids replace sediments with silica.If water is the transport medium of sediment, the grain size of sedimentary deposits most closely indicates the _____. a. geographic extent of the weathering source rock at outcrop b. average velocity of the water from the time of erosion until deposition c. velocity of the water at the moment the sediment settled to the bottomArbitrarily defined limits for the sizes of crystals within an igneous rock. Commonly accepted grain-size ranges are:For sedimentary rocks, see particle size. Very coarse. >3 cm. Coarse. 5 mm to 3 cm. Medium. 1–5 mm. Fine.Whole grains are an excellent source of complex carbohydrates and fiber, making them the healthiest type of grain a person can eat. In fact, the Dietary Guidelines for Americans suggest that whole grains should make up at least 50% of all t...Chert It has the same chemical formula as quartz, but is cryptocrystalline , meaning that the quartz crystals comprising chert are so small that it is difficult to see them even under a microscope. Chert can be a chemical sedimentary rock, often forming as beds within limestone (Figure 9.14), or as irregular lenses or blobs (nodules).

Jun 20, 2019 · Gravel forms coarse rocks with grains over 2 mm in size. If the fragments are rounded, they form conglomerate, and if they are angular, they form breccia. Sand, as you may guess, forms sandstone. Sandstone is medium-grained, meaning its fragments are between 1/16 mm and 2 mm. Silt forms fine-grained siltstone, with fragments between 1/16 mm and ... Classification of Sedimentary Rocks from the Mode of Origin: 1. Clastic Rocks: These are made of rock fragments or grains of minerals broken from any type of pre-existing rock. These are classified based on the sizes of the fragments. Sedimentary rocks containing extremely large grains are called conglomerates if the clasts are rounded, and ...The burning meteor that causes a shooting star can vary from the size of a grain of sand to a 1-meter-long object. As the meteor falls through the atmosphere, it burns up and gives the appearance of a star falling.Grain size – Variable, can consist of clasts of all sizes. Hardness – Generally hard. Clasts – if clastic / bioclastic then grains and / or broken or whole shell fragments visible; if non-clastic / chemical then crystalline and no clasts visible. Major minerals: Calcite, dolomite. Geological formation and occurrence of limestoneGrain size: > 2mm; Clasts easily visible to the naked eye, should be identifiable. Hardness: Soft to hard, dependent on clast composition and strength of cement. Colour: variable, dependent on clast and matrix composition. Clasts: variable, but generally harder rock types and / or minerals dominate.Chert Add ⊕ 1 Properties 1.1 Physical Properties 1.1.1 Hardness 6.5-7 1.1.2 Grain Size Very fine-grained 1.1.3 Fracture Uneven, Splintery or Conchoidal 1.1.4 Streak White 1.1.5 …

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Feb 13, 2023 · Chert breccia. Rock Type: Sedimentary – A poorly sorted and immature detrital (clastic) sedimentary rock made up of angular rock fragments that are gravel sized or larger (2mm or more). Click on image to see enlarged photo. One pound of uncooked long grain rice contains more than 29,000 grains. In the U.S., rice is sold in bags of 2, 5, 10 and 50 pounds. Rice has more than 40,000 species, though only a few varieties are marketed for sale.Grain size is the average diameter of sediment fragments in sediment or rock. Grain sizes are delineated using a logbase-2 scale [ 9 ; 10 ]. For example, the grain sizes in the pebble class are 2.52, 1.26, 0.63, 0.32, 0.16, and 0.08 inches, which correlate respectively to very coarse, coarse, medium, fine, and very fine granules. e. Wentworth grain size chart from United States Geological Survey Open-File Report 2006-1195: Note size typos; 33.1mm is 38.1 & .545mm is .594. Beach cobbles at Nash Point, South Wales. Grain size (or particle size) is the diameter of individual grains of sediment, or the lithified particles in clastic rocks.

These properties make it useful as a filter media, an absorbent, and as a lightweight filler for rubber, paint, and plastics. When diatomite is crushed into a powder, it is usually called "diatomaceous earth," or D.E. Diatoms are microscopic, single-celled algae that live in marine or fresh water. They produce hard parts made of silicon dioxide.This produces a non-biogenic chert. There are many varsities of such chert that are given different names depending on their attributes, For example: Flint – Black or gray from organic matter. Jasper – Red or yellow from Fe oxides. Petrified wood – Wood grain preserved by silica. Agate – Concentrically layered ringsParticle size indicates the energy of the transporting medium. The larger the size of grains in a clastic rock, the more energy it took to move that particle to the place of deposition! 2) Angularity: Degree to which the individual sedimentary particles are rounded. WELL ROUNDED - all corners of a grain are rounded offsedimentary rock. Sedimentary rock - Clastic, Chemical, Organic: There are many different systems of classifying sandstones, but the most commonly used schemes incorporate both texture (the presence and amount of either interstitial matrix—i.e., clasts with diameters finer than 0.03 millimetre—or chemical cement) and mineralogy (the ...Chert Composition. Chert is mainly composed of silicon dioxide microcrystals. These silicic components may have come from accumulated particles of other rocks brought by water or wind. Also ...Grain size: no grain size Components: biochemical/organic Minerals: calcite, with some shells and skeletal fragments Transportation: water. Siltstone. Grain size: < 1/16 mm ... Chert. Grain size: no grain size Components: chemical Minerals: silica (quartz) Transportation: wind. Sandstone. Grain size: 1/16 - 2 mmIn the chert of Xionger Group, there were clear banded (or lamellar) structures which were contributed by the diversities of the grain size and mineral composition. The different bands (or lamellars) had alternative appearance repeatedly, and denoted the diversities and periodic changes in the substance supply during the precipitation.... particle-size as well. The rock fragments are dominantly angular chert pebbles or cobbles. In some places the fragments are sub-rounded. Some limestone ...The type of rock that the fragments were produced from is often used as an adjective when referring to the rock. Some examples: sandstone breccia, limestone breccia, granite breccia, chert breccia, basalt breccia, and others. Often a breccia will contain many types of angular rock fragments. These are known as polymict breccias or polymictic ...Sep 8, 2023 · Grain Size. Most chert is microcrystalline, meaning the individual quartz crystals are under 10-50 microns in size and indistinguishable to the naked eye. Some varieties like flint are cryptocrystalline, with even smaller grains under 5 microns. Quartz. Quartz makes up over 90% of chert in most cases. Nonfoliated textures are further described on the basis of the grain (crystal) size in the rock. Examples of complete descriptions of nonfoliated metamorphic rocks include nonfoliated, medium grained for quartzite, or nonfoliated, coarse grained for marble. ... Composed of rock fragments, quartz, chert. Conglomerate: Meta-conglomerate: Very ...Sedimentation means settling from a fluid, either water or air. The texture of sedimentary rocks is described as "clastic", meaning that they are composed of discrete particles that are compacted and cemented together. An example of a non-clastic texture would be crystalline material. Detrital Sedimentary Rocks. Sediment name. and particle size.

The name "flint" is often the preferred name used for material with a very fine grain size and a slightly higher luster. These "fine-grained" materials break with greater predictability and produce a sharper edge. Many ancient tool makers understood the quality and properties of the materials that they used.

If water is the transport mechanism of sediment, the grain size of sedimentary deposits most closely indicates the _____. a. geographic extent of the weathering source rock at outcrop b. average velocity of the water from the time of erosion until deposition c. velocity of the water at the moment the sediment settled to the bottom d. climate conditions at the time of depositionThese properties make it useful as a filter media, an absorbent, and as a lightweight filler for rubber, paint, and plastics. When diatomite is crushed into a powder, it is usually called "diatomaceous earth," or D.E. Diatoms are microscopic, single-celled algae that live in marine or fresh water. They produce hard parts made of silicon dioxide.Physical properties of rocks are used to identify the type of rocks and to discover more about them. There are various physical properties of Flint like Hardness, Grain Size, Fracture, Streak, Porosity, Luster, Strength etc which defines it. The physical properties of Flint rock are vital in determining its Flint Texture and Flint Uses. Study Minerals flashcards. Create flashcards for FREE and quiz yourself with an interactive flipper.A cup of plain, uncooked rice contains anywhere from 5,000 to 10,000 grains. The number of grains found in a cup of rice varies considerably by the variety of rice, as there are over 40,000 types of rice throughout the world, each of varyin...Texture (grain size) Coarse (over 2 mm) Medium (1/16 to 2 mm) Fine (1/16 to 1/256 mm) Very Fine (less than 1/256) Composition Calcite Quartz Gypsum Halite Altered plant fragments Clastic Sedimentary Rocks Sediment Name ...Size Grades (mm): 2-4 granules; 4-64 pebbles; 64-256 cobbles; >256 boulders: Composition of Major ...

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Chert Grain Size (chemical) Sorting Roundness Layered? Fossils? Acid Reaction? Conglomerate Grain Size (clastic) Sorting Roundness Layered? Show transcribed image text.Clastic sedimentary rocks are subdivided on the basis of grain size. Secondary characteristics used to further distinguish clastic rocks are sorting, grain shape, and grain composition. Grain size: Clastic sediments are distinguished on the basis of average grain size. gravel-size particles are larger than 2 mm diameterChert can be almost any color but is most commonly grey, brown, reddish, or light green. Most varieties are opaque, but it can also be translucent. Pro Tip: I have created the best rock identification system you'll find anywhere.Get ratings and reviews for the top 10 gutter companies in Grain Valley, MO. Helping you find the best gutter companies for the job. Expert Advice On Improving Your Home All Projects Featured Content Media Find a Pro About Please enter a va...Grain Size: Coarse grains are visible to the naked eye, and the minerals can usually be identified without using a magnifier. Fine grains are smaller and usually cannot be identified without using a magnifier.; …Study with Quizlet and memorize flashcards containing terms like Texture: Clastic (course) Composition: Chert Grain size: sand/silt-gravel (mostly gravel) sorting: poorly sorted roundness: angular Fossils: N/A Identifying markers: scratches glass, large angular chunks, Texture: Clastic (course) Composition: Quartz (Iron oxide?) Grain size: sand-pebble (mostly pebble) sorting: poorly sorted ... Chert Grain Size (chemical) Sorting Roundness Layered? Fossils? Acid Reaction? Conglomerate Grain Size (clastic) Sorting Roundness Layered? Show transcribed image text.6.1 Clastic Sedimentary Rocks. A clast is a fragment of rock or mineral, ranging in size from less than a micron [1] (too small to see) to as big as an apartment block. Various types of clasts are shown in Figure 5.12 and in Exercise 5.3. The smaller ones tend to be composed of a single mineral crystal, and the larger ones are typically ... ….

This observation is non-destructive and fast, and makes it possible the grain ... Thin sections show these cherts are microcrystalline with length-fast chalcedony ...Chert is a biochemical sedimentary rock made from microcrystalline quartz. It is very hard – registering a 7 on the Mohs hardness scale, and displays conchoidal fracturing. Chert can almost any color, but is usually gray, brown, red, or green. It has many subvarieties including flint, jasper, agate, and onyx.Clastic sedimentary rocks may have particles ranging in size from microscopic clay to huge boulders. Their names are based on their clast or grain size. The smallest grains are called clay, then silt, then sand. Grains larger than 2 millimeters are called pebbles. ... Chert is an example for this type of rock, and this is one of the ways ...Sedimentary rock - Iron-Rich, Clastic, Chemical: Almost all sedimentary rocks are iron-bearing in the sense that mudrocks, sandstones, and carbonates typically have an iron content of several percent. Nevertheless, sedimentary rocks in which the proportion of iron exceeds 15 percent are separately categorized as iron-rich. Two major types of iron-rich …Science. Earth Sciences. Earth Sciences questions and answers. DQuestion 1 3 pts Match the physical properties to the appropriate rock name Breccia IChoose ] I Choose ] GRAIN SIZE: sand to pebble sized; SORTING: poorly sorted; ROUNDNESS: rounded to subrounded; LAYERED: no; FOSSILS: no; ACID REACTION: no GRAIN SIZE: sand to …Chert. Grain Size: Microcrystalline Grain Composition: Chemical Grain Sorting: N/A Mineralogy: Quartz Structures or fossils: ... N/A Mineralogy: Gypsum Structures or fossils: Soft, clear evaporite. Dolomite. Grain Size: Silt and Sand Grain Composition: Chemical Grain Sorting: Good Mineralogy: Dolomite Structures or fossils: Fizzes only if ...Size Grades (mm): 2-4 granules; 4-64 pebbles; 64-256 cobbles; >256 boulders: Composition of Major Fraction: Volcanic Ejecta (also in grain size 1/256-2 mm category) Chiefly One Constituent Especially quartz, chert, or quartzite. Also shale or limestone. Homogeneous conglomerates and breccias. Several Constituents Usually including unstable The width of the sample is 16 cm. Banded iron formations formed in Precambrian time. The formation of BIF is usually associated with the oxygenation of seawater by photosynthetic cyanobacteria, one of the …If water is the transport mechanism of sediment, the grain size of sedimentary deposits most closely indicates the _____. a. geographic extent of the weathering source rock at outcrop b. average velocity of the water from the time of erosion until deposition c. velocity of the water at the moment the sediment settled to the bottom d. climate conditions at the … Chert grain size, The name "hornfels" is assigned to a rock after considering its grain size, texture, and geologic history. As a result, hornfels does not have a specific chemical or mineralogical composition. It inherits its composition from the rocks that are metamorphosed plus the fluids involved in the metamorphic process., Grain Size Quartz (± cement) Quartz + Feldspar (± cement) Quartz +Clay Matrix (± cement) Calcite Grains (not inter grown crystals) and calcite cement Calcareous Fossil fragments + calcite cement (± calcite grains) Gravel Coarse grained Majority of grains over 2 mm. in di ameter Quartzose Conglomerate if fragments are rounded Quartzose ..., For example, the portion of the key below requires a number of observations: grain size, conchoidal fracture, hard enough to scratch fingernail, etc. The degree ..., The problems with chert come in its origin; it’s another one of those rocks whose origin we have a hard time observing directly. The relative importance of nodular chert and bedded chert has changed through geologic time: bedded chert is much more common in the Precambrian, and nodular chert is more common in the Phanerozoic. 1.2 Terminology , Classification of Sedimentary Rocks from the Mode of Origin: 1. Clastic Rocks: These are made of rock fragments or grains of minerals broken from any type of pre-existing rock. These are classified based on the sizes of the fragments. Sedimentary rocks containing extremely large grains are called conglomerates if the clasts are rounded, and ..., chert and flint, very fine-grained quartz (q.v.), a silica mineral with minor impurities. Several varieties are included under the general term chert: jasper, chalcedony, agate (qq.v.), flint, porcelanite, and novaculite. Flint …, Chert is a biochemical sedimentary rock made from microcrystalline quartz. It is very hard – registering a 7 on the Mohs hardness scale, and displays conchoidal fracturing. Chert can almost any color, but is usually gray, brown, red, or green. It has many subvarieties including flint, jasper, agate, and onyx., Expert Answer. 1. As the rock is made up of powdery limestone, the rock is "Chalk". Sandstone is made up of quartz and feldspar. Shale is made up of quartz, feldspar and clay minerals. Chert is made up of microcrystalline quartz. So the answer is " Chal …. Knowns: This rock is a powdery limestone containing calcite from the shells of ..., The critical independent variables controlling porosity (dependent variable) are detrital composition, sorting, time-integrated temperature history, and time-integrated pressure history. In sandstones with a high content of detrital quartz and chert, grain size may also affect porosity., According to Fisher (1958) the size of fragments present in a breccia is an important parameter, which varies from 2 mm to >64 mm (Table 1). Based on the grain size limits, he has grouped volcanic breccia into four classes. The grain/fragment size in class-1 ranges between 2 and 4 mm and termed as Tuff, in class-2 it is between 4 and 32 mm …, The definition of chert itself indicates about grain size range from very fine to cryptocrystalline; hence, granularity is not useful for general classification of chert. …, (chert) Deep seafloor where plankton with microscopic SiO 2 shells rain down. Detrital sedimentary rock (clasts are mostly rock and/or mineral fragments Grain size Texture and composition Name Depositional environment Gravel Rounded fragments; poorly sorted Conglomerate Beach headlands, natural levees, tops of alluvial fans., The grain sizes of sediments and sedimentary rocks are a matter of great interest to geologists. Different size sediment grains form different types of rocks and can …, Oct 21, 2023 · A. A well-sorted clastic rock is made up of different-sized particles. B. Conglomerates consist of a single grain size. C. An immature clastic rock has well-sorted, resistant grains. D. Pieces of broken rock produced by physical weathering are collectively called clasts or detritus. D. Pieces of broken rock produced by physical weathering are ... , Detrital rocks are classified by grain size with some subdivisions based on the composition and the shape of the detrital particles. ... Chert is a commonly chemical sedimentary rock that forms from the precipitation of silica (microcrystalline …, You might think the only differences between whole wheat and white bread are superficial things, like texture and taste. But it’s also true that whole wheat bread gets touted as the healthier option., - Fizzes with acid - Rock Type = Chemical - Grain size = Gravel, Sand - Sorting & Rounding = N/A - Composition = Fossils, Carbonate - Rock Type = Biochemical - Other notes: visibly see fossils Chert - Rock Type = Chemical - Grain size = Microcrystalline - Sorting & Rounding = N/A - Composition = Silica, Quartz - Rock Type = Chemical ..., Granular microquartz is the component of chert consisting of roughly equidimensional quartz grains, ranging in size from a fraction of a micron to 20 microns, but most typically 8 to 10 microns. Chalcedony is a microfibrous variety of quartz, consisting of radiating bundles of very thin crystals about 100 microns long., Study with Quizlet and memorize flashcards containing terms like q3: Coal is a form of, q4: Which of the following statements is true? A. Pieces of broken rock produced by physical weathering are called clasts. B. A well-sorted clastic rock is made up of a wide range of particle sizes. C. Conglomerate consists of a single grain size. D. All well-rounded clasts are also spherical., q5: A ..., Grain silos are an essential component of the food supply chain, playing a critical role in storing and preserving grains until they are ready for consumption. These towering structures, which can reach up to 100 feet tall, are commonplace ..., Properties of Chert Chert Add ⊕ 1 Properties 1.1 Physical Properties 1.1.1 Hardness 6.5-7 1.1.2 Grain Size Very fine-grained 1.1.3 Fracture Uneven, Splintery or Conchoidal 1.1.4 Streak White 1.1.5 Porosity Highly Porous 1.1.6 Luster Waxy and Dull 1.1.7 Compressive Strength Properties of F.. ⊕ 450.00 N/mm 2 Rank: 1 (Overall), Chert: Clastic Sedimentary Rocks. How Clastic Sedimentary Rocks are Formed: Pre‑existing rock undergoes chemical and mechanical weathering by roots, acid rainwater, gravity, wind, and water. ... Grain size and shape and composition can indicate the composition, distance, and height of the source rock. Textures & structures (ripple marks ..., Sand is loose, non-cohesive granular material and term is defined by its size. Grain size ranging from 0.0625 (1/16) mm to 2mm in diameter. Sandstone is lithified rock as a result of diagenesis. ... quartz grains, chert grains and grains made up of quartz and rounded secondary overgrowth that can be recognised as second cycle grains. Quartz is ..., Grain size is the primary parameter in sedimentary geology to divide clastic rocks and sediments in different classes for classification purposes. This subdivision is based on the Udden-Wentworth scale, which differentiates four major classes of sediments (and corresponding sedimentary rocks) that are further divided in sub-classes:, The phi scale allows more emphasis for the finer grain sizes. Phi size values for the sediment class limits range from −5 phi (for a diameter of 32 mm, or very coarse pebble size) down to +10 phi (for a diameter of 1/1,024 mm, or clay size). The scale was developed specifically as a statistical device to permit the direct application of ..., ... grain size of archaeological lithic is much smaller and equals to 41.9 μm. Average grain size of chert could not be measured due to the cryptocrystalline ..., Sandstones are sedimentary rocks that can be made up of any sand-size particles. In Mississippi, most sandstones are made up of quartz grains. Sandstones can be cemented to various degrees of competence by a few different methods. Types of cement such as opal, chert, chalcedony, and quartz typically form the strongest bonds between the …, Grain size: Cryptocrystalline, cannot be seen except under very high magnification. Hardness: Hard. Colour: All colours, dependent on impurities present when precipitated. Clasts: None. Other features: Smooth to touch, glassy, exhibits conchoidal fracture. Occurrence of Chert, Sedimentary rocks are classified according to the predominant grain size present, as well as by their mineral content. This website uses cookies to ensure you get the best experience on our website. ... Chert is a fine-grained hard sedimentary rock composed of microscopic silica grains, and has a flat fracture. Flint is a variety of chert ..., The name "flint" is often the preferred name used for material with a very fine grain size and a slightly higher luster. These "fine-grained" materials break with greater predictability and produce a sharper edge. Many ancient tool makers understood the quality and properties of the materials that they used., Clastic sediment is the solid bits and pieces of rocks and minerals that have broken down from the source rock by a combination of physical and chemical weathering. Clastic sediment is sometimes called detrital sediment. Grains of clastic sediment range in size from microscopic (clay particles) to house size (large boulders), with sand and silt ..., The size of the grains selected for sample preparation depends on the purpose of the experimental investigation. In this research, to study the effect of grain size on sandstone properties, a wide range of grain size was used. The median grain size, D 50, (the mesh size where 50% of the grain pass through) was selected to be between …, This observation is non-destructive and fast, and makes it possible the grain ... Thin sections show these cherts are microcrystalline with length-fast chalcedony ...