Silt: Grains of silt size, chiefly quartz, but may be composed entirely or 

 partly of other minerals. 



Skeletal: See "dolomoldic" and "oomoldic." 



Smooth: Major type of chert with conchoidal to even fracture; surface 

 devoid of roughness ; may be botryoidal ; homogeneous ; no distinctive structure, 

 crystallinity, or granularity. 



Speckled: Disseminated fine spots of color or material different from that 

 of the matrix and having relatively sharp boundaries. 



Spicular: Containing inclusions of sponge spicules. Free spicules have 

 been noted. 



Subhedral: Crystal forms partly developed; may be loose, drusy, or granu- 

 lated. 



Subrounded: Polygonal grains or fragments but with well-rounded edges 

 and corners. 



Unmodified: Residue uniform with no modifying characteristics. 



The most common residues are chert and sand, with chert rated as the 

 most diagnostic. Texture, color, transparency, luster, and crystallinity are the 

 chief factors for the differentiation of chert. Inclusions and modifying character- 

 istics are secondary factors. Chalcedonic and ordinary chert are the most 

 abundant of the smooth cherts. The term granular chert is applied to obviously 

 crystalline chert or that with observable grains. Smooth and granular cherts 

 grade into each other and into chalky chert. The chalky types are those of which 

 the original internal structure and filled interstices have been affected by weath- 

 ering and probably by circulating water. Tripolitic chert when placed in acid 

 leaves a very fine, porous, chalky chert because of the solution of disseminated 

 calcium carbonate. All the cherts may be dolomoldic, the dolomolds ranging 

 in type from scattered to skeletal and in size from fine to very large. 



The color of chert is an important diagnostic feature. It is prevalently 

 colorless, white, gray, tan, and brown, but all colors are found. Many residues 

 from beds in Missouri, Kansas, Oklahoma, and Texas have sudden color changes 

 which mark boundaries of zones or formations. The smooth brown chert of the 

 Lower Devonian in West Texas is difficult to differentiate from that in the Upper 

 Ordovician, and drilling to an underlying boundary is necessary in many places 

 for positive identification. 



Organisms may be replaced by silica or other insoluble matter and may be 

 identified in the residues, especially small forms and foraminifera, which 

 generally are not broken by the drill. Molds of organisms are common where 

 soluble shells or fragments have been imbedded in an insoluble matrix. Beekite 

 occurs most commonly in replaced megascopic fossils found in outcrop samples. 



Quartz may be euhedral and authigenic or subhedral and anhedral from 

 veins, cavity filling, or interstitial openings. Quartz sand of various types from 

 rounded to angular may be found as scattered inclusions or as a dominant 



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