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 distorted and interlocked 

 with adjacent grains in such a manner that a tight, nonporous formation results. 

 Feldspar, mica, glauconite, and other minerals are common as residue con- 

 stituents of sandstone, althought 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 Upper 

 Cambrian. Few of the Lower Ordovician Beekmantown beds have glauconite, 

 and the appearance of glauconite generally marks the top of the Cambrian. 



Pyrite is a common insoluble residue seen as small to large, euhedral 

 crystals in limestone, dolomite, and shale. It also occurs spongelike, disseminat- 

 ed, and in veins and cavities. Pyrite has little value as a diagnostic residue, but 

 it has a secondary value as an inclusion in chert or shale. When pyrite occurs 

 in abundance, it may serve as a marker bed and often identifies a zone of cir- 

 culating water or an unconformity. 



Interstitial spaces due to primary or secondary permeability, alteration, 

 or replacement in calcareous rocks may become filled with silica, pyrite, or other 

 insoluble material. Solution of the matrix leaves fragile, lacy networks that are 

 generally destroyed by acid effervescence and washing. These residues are the 

 extreme upper limit of skeletal dolomolds, pyrimolds, and oomolds. Residues 

 from veins or fractures are curved or tabular flakes. Vein fillers or cement for 

 brecciated residues include gilsonite, silica, pyrite, and sphalerite. 



Siliceous limestones have residues that are generally earthy, finely porous, 

 and dark-colored. These residues are especially noteworthy because examination 

 of such samples before solution gives no clue to the type of residue. The residues 

 from siliceous limestone also appear to be 100-percent insoluble by volume, but 

 they may be 50-percent insoluble by weight, owing to the removal of the inter- 

 stitial lime. 



Siliceous oolites are common and may be found free, clustered, or in a 

 matrix. An oolite, to be identified as such, must have a nucleus and at least one 

 concentric layer or shell. Nuclei may range in size from very minute to one 

 occupying nearly all of the interior mass. Most ooliths have several shells. 

 Ooliths are classified according to the interior structure as concentric, massive, 

 radiate, or sand-centered. Clustered or free ooliths may be frosted with a crust 

 or minute drusy quartz or may have a smooth, siliceous shell. Silica may replace 

 calcareous ooliths and cause them to be preserved as residues. Ooliths have many 

 colors and in many instances occur embedded in different-colored matrices. All 

 types of chert have ooliths, although in chalky chert they are rare. 



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