164 THE AMERICAN MONTHLY [June 



noidal limestone. The crystals making up the remainder of the 

 i-ock are calcite. Almost all of the sections contain some frag- 

 ments of crinoids, and this was an age of crinoids, in distinction 

 from the lower group. 



Niagara Series. — Consisting of shales and a magnesian lime- 

 stone, with considerable fossil life, which is mainly preserved in 

 the form of internal casts, the characteristic forms being Pen- 

 tamerus^ Trimerella^ JSlonotnerella^ and Atrpya. This marks 

 the second great limestone formation in the scale, and doubtless 

 was deposited in the Niagara sea as a pure carbonate of lime, 

 but, by a slow dolomitic replacement, was changed into a fairly 

 pure dolomite, and now is used quite extensively as a source of 

 magnesian lime. From these focts it will be seen that a micro- 

 scopical study will not be as satisfiictory as heretofore, for but 

 little original fossil matter is left. The dolomite crystalsare very 

 apt to break out in the process of grinding, thus leaving open ir- 

 regular spaces through the section. 



Lower Helderberg Formation, — or waterlime group of 

 Ohio, which consists of a magnesian limestone, inclosing at a 

 number of points large beds of rock salt and gypsum. 

 This formation covers two dozen counties of the State, and the 

 limestone contains a notable percentage of bituminous matter, 

 appearing in streaks through the rock, which accounts for the 

 odor of petroleum when it is struck with the hammer. It has 

 long been regarded unfossiliferous, with the exception of a few 

 casts, the most common being the small coftee-grain fossil, Lep- 

 erditia alta. In iSSo some blocks of this limestone were pol- 

 ished for the National Museum, and it was stated that '•'• one pol- 

 ished upon the surface parallel to the plate of stratification showed 

 a fossil bryozoan, and thus proved that it was fossiliferous." 

 We are forced to say that the Helderberg limestone of Ohio is 

 very sparingly fossiliferous, even in microscopic sections, but we 

 cannot say the Helderberg sea in Ohio was without life. 



Devonian, or Coniferous. — Extends as a narrow strip, S-30 

 miles in width, through the central portion of the State. The 

 limestone is quite a pure carbonate, abundantly fossiliferous, con- 

 taining brachiopods and reef-building corals in great abundance. 

 It is here that we find the first abundant plant and fish life pre- 

 served. 



At Columbus probably the best development is to be found. 

 The upper portion is shaly, with very few fossils, and these are 

 mainly fish remains ; below this stratum comes a brown and 

 white chert, which is not acted upon by acids. The fossils in 

 this chert are very finely preserved, even the delicate markings 

 coming distinctly to view. Below this comes the bone bed, a 

 comparatively thin layer made up of the teeth and remains of 

 fish, while below this lies the great mass of limestone used for 

 lime and building purposes. The microscope reveals many in- 

 teresting and puzzling fossil forms. 



