1893.] MICROSCOPICAL JOURNAL. 163 



Microscopical Study of Ohio Limestone, 



By G. perry GRIMSLEY, 



COLUMBUS, OHIO. 



[Abstracted from Journal of Cincinnati Society of Natural History, 

 vol. XV, pp. 160-167.] 



The year 1S50 may be said to mark the beginning of modern 

 petrography, when Mr. H. C. Sorby, of Enghand, appHed the 

 mici'oscope to the examination of thin rock sections. American 

 students brouglit the science across the water, and now we have 

 the names of Williams, Iddings, Cross, Wadsworth, Adams, and 

 others connected with the science. 



Lower Silurian Series. — The remnants of tiie old Silurian 

 or Ordovician sea in Ohio occupy about a dozen counties in the 

 southwestern corner of the State. The beds are highly fossilif- 

 erous, containing large numbers of brachiopods, lamellibranchs, 

 trilobites and crinoids, so that the region forms the best collecting 

 ground in the State for the paleontologist. 



Selecting carefully a dozen slides, and examining them imder 

 the microscope, we find revealed to the eye monticuliporoids, with 

 the species ruammtilata and ranwsa especially abundant ; the 

 Trenton tentaculite ; the spiny head-shield ofacidaspis; crinoid 

 joints very numerous, and with these fragments of shells, with their 

 characteristic structure ; of these only one was perfect enough for 

 identification — the small Cyclora mimita. Numerous crystals 

 of calcite or dolotnite occur, showing the characteristic rhombo- 

 hedral cleavage. It is impossible to sepax^ate these two minerals 

 under the microscope, but, as this is a magnesian limestone, the 

 crystals might be termed dolomite. A very fine section of Ptilo- 

 dictya fcnestellifor7nis was observed, which did not appear be- 

 fore the grinding, thus showing the utility of microscopic exam- 

 ination of rocks. 



The predominant structure consists of shells and coral re- 

 mains, either entire or broken. These shell fragments, though 

 too small for identification, are yet useful to show the minute 

 shell structure, appearing under a low power as mere dots, but 

 under a high power they are resolved into obscure wavy lines 

 and circles. 



Clinton Group. — A narrow fringe forming the boundary be- 

 tween the lower Silurian and Niagara formations. The forma- 

 tion is of small extent, and in composition is almost a pure car- 

 bonate of lime. The limestone is distinctly fossiliferous, consist- 

 ing of brachiopods, corals, and crinoids. Thin sections show 

 many fragments of Clathropora^ probably the clintonensis^ a few 

 fragments of Favosites, with other corals and a few shells. One 

 layer of a light brown color, probably colored by iron, is quite 

 compact, semi-crystalline, and only sparingly fossiliferous. 

 Another form of the rock is of a light color, crystalline, and made 

 up almost entirely of fragments of crinoids, being a typical cri- 



