148 Subsurface Geologic Methods 



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 modi- 

 fying characteristics 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 weathering and probably by cir- 

 culating water. Tripolitic chert when placed in acid leaves a very fine, 

 porous, chalky chert because of the solution of disseminated calcium car^ 

 bonate. 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 diflScult to differen- 

 tiate 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 com- 

 mon where soluble shells or fragments have been imbedded in an insolu- 

 ble 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 feature in a sandy calcareous rock. Secondary enlargement 

 or regrowth of quartz crystals around sand grains is a common occurrence 

 in the Lower Paleozoic. The crystal growth in some sandstones is dis- 

 torted and interlocked with adjacent grains in such a manner that a tight, 

 nonporous formation results. Feldspar, mica, glauconite, and other min- 

 erals are common as residue constituents of sandstone, although quartz 

 is the chief constituent. Calcareous material interstitially mixed with very 

 fine quartz in silt and clay sizes results in a very fine porous residue. 



Glauconite is abundant in sands and is scattered throughout many cal- 

 careous beds. It is a good marker for many beds in the Paleozoic, chiefly 

 in the Mississippian, Middle Devonian, Middle and Lower Silurian, and 



