18 



valves, and are at once recognized by their semi-elliptical 

 outline, slight convexity and straight hinge line. 



These three species occupy the rock almost to the exclusion 

 of every other form, and constitute a distinct association of 

 fossils, which is characteristic of the upper part of the 

 Moscow shale of this region. The fauna thus produced con- 

 stitutes the "Schizobolus fauna," named so after its most 

 characteristic member, and, inasmuch as it contains typical 

 Hamilton and typical Genesee fossils, it is a true transition 

 fauna from the Middle to the Upper Devonian of this region. 



The most fossiliferous portion of the rock is that portion 

 having the character of individual concretions. The more 

 continuous portion of the bed, while containing these fossils, 

 is nevertheless comparatively barren. 



The limestone rests on gray calcareous sha 1 ^, readily split- 

 ting into thin layers, and moderatelv fossiliferous. On the 

 surfaces to which air and water have access whitish or 

 vellowish granules can usually be observed scattered thickly 

 over the shale and the fossils. Sometimes these are so closely 

 crowded as to give the rock an oolitic appearance. Under a 

 lens these granules appear dull, rounded or disclike, but 

 under a microscope they appear to be bunches or aggregates 

 of small crystals. Analysis shows them to be crystals of 

 gypsum (hydrous sulphate of calcium). The origin of these 

 crystals is explained by the occurrence of pyrite grains and 

 nodules in considerable number in the shale. These by 

 oxidation form sulphate of iron, which reacts with the 

 calcium carbonate in the shale and produces calcium 

 sulphate. Free sulphuric acid is likewise formed, which 

 reacts with the calcium carbonate to form calcium sulphate, 

 water and carbon dioxide. The calcium sulphate, from the 

 presence of water during its formation will be hydrated. 

 The formation of the gypsum is probably going on con- 

 stantly, just as the alum is constantly forming on the ex- 

 posed laminae of the Genesee and other bituminous shales.* 



•See the reactions given in Chapter II. 



