PALE0NT0L0GICAL REPORT OF GEOLOGICAL SURVEY. 51 9 



on a large scale, and follow the same rules as the shales. They may 

 also thicken, or entirely disappear, or accidentally separate into two or 

 three branches. 



The shales may be wanting, either from erosion, or from the uphea- 

 val of a part of the surface above the water, or from the more active 

 growth of the vegetable matter in a peculiar spot. The two last caus- 

 es are scarcely observed: the first and more frequent one shall be 

 mentioned again with the sandstone. Generally speaking, the absence 

 of thft shales is local, and ordinarily, even where they seem to be en- 

 tirely wanting, if the mine be worked to any extent, they are discov- 

 ered in some places. 



4th. THE LIMESTONE. 



This formation can be regarded as a continuance, and sometimes as 

 an equivalent, of the shales, since it is established only in an undis- 

 turbed sheet of deep marine water, by the continuous labor of marine 

 animals, especially moluscs, and by the decomposition and accumula- 

 tion of their remains. The essential reason of its formation, viz: 

 deep, quiet, marine water, is nevertheless a cause of great variety — 

 not in its chemical elements, perhaps, but in its persistency, its thick- 

 ness, and its general distribution. It is often found in the Coal Meas- 

 ures in an unfinished state, in irregular masses, which can scarcely 

 take the name of beds, so limited are they. For this reason the lime- 

 stone, by its presence above a bed of coal, is scarcely a reliable guide 

 for identification. 



As long as the shales of the coal were deposited in low water, the 

 influence of the sea, especially its currents, were scarcely appreciable. 

 But in the limestone formation it is very visible indeed. The unequal 

 distribution of the matter, and especially the remarkable erosions of 

 the beds or isolated masses of it, are due to slow currents. 



The limestone of the western coal-fields of America, contain a great 

 amount of organic remains, plants, shells, or fishes. But the plants 

 cannot give a reliable criterion for the geological level of each peculiar 

 strata, since all the remains found till now are only broken, deformed, 

 and undeterminable parts of stems, with few marine fucoids. The re- 

 mains of shells and fishes would probably afford some reliable data for 

 tracing the geological level of the beds of limestone. They are only 

 too numerous in their species, and have neyer been subjected to a 



