Subsurface Laboratory Methods 149 



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, dis- 

 seminated, and in veins and cavities. Pyrite has little value as a diag- 

 nostic 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 circulating water or an unconformity. 



Interstitial spaces due to primary or secondary permeability, altera- 

 tion, 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 wash- 

 ing. 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 gil- 

 sonite, silica, pyrite, and sphalerite. 



Siliceous limestones have residues that are generally earthy, finely 

 porous, and dark-colored. These residues are especially noteworthy be- 

 cause 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 interstitial 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 presjerved as residues. Ooliths have many colors and frequently 

 occur embedded in different-colored matrices. All types of chert have 

 ooliths, although in chalky chert they are rare. 



Cherjt in many cases has included sand grains, which may be con- 

 fused with ooliths. Shells are absent, however, and the clear quartz of 

 the said grain may be observed. 



Pseudoolites or "shadow oolites" resemble oolites and may resemble 

 included quartz sand grains. The boundary between the matrix and the 

 oolith is indistinct, however, and the central portion, which cannot be 

 identified as quartz is only a shade lighter or darker than the other por- 

 tions. Pseudoolites may be ooliths or sand grains that have been resorbed, 

 thus destroying any formerly existing boundaries. 



Dolomolds occur chiefly in chert residues from dolomites, rarely in 

 chert from limestone. Dolomolds are common in shale residues and are 



