SCIEXTIFIC LeCTUBF.S. 85 



extent and thickness, and these indicate a depression of the ocean 

 Led and a quiet sea. In the Coal measures, \\q are able to demon- 

 strate in the most satisfactory manner the gradual encroachment of 

 the coast line upon the area of the ancient ocean. We find layers of 

 clay and sand pushing out to the westward, overlying the limestones 

 which have been deposited in a quiet sea. These beds of sand, clay, 

 or coarser materials, are charged with the remains of plants which 

 jnust have grown upon or near the place where we find them 

 imbedded, and in some instances, the roots remaining in the soil upon 

 whicli they grew and flourished. Such conditions can have come 

 only from the gradual elevation of the ocean bed, allowing that part 

 so lately covered with its waters to become extensive marshes and 

 moderately elevated areas of dry hmd, which supported this growth 

 of vegetation. 



During the latter part of this period the dry land has extended 

 as far westward as Missouri and Kansas, as we have evidence in the 

 extension of the Coal measures. It is in this region that the Coal 

 measures, or coal-bearing portions of the great Carboniferous system 

 of rocks thin out, and lose the character which has given tliem their 

 designation ; and this system of rocks gives us one of the best 

 examples of the thinning out of sedimentary strata which we have in 

 the whole series. On the nortlieast coast in Xova Scotia, the coal 

 formation has a measured thickness of from fourteen to sixteen 

 thousand feet, while the same formation on the west of the Mississippi 

 has a thickness of a few hundred feet only. But it must not be 

 understood that the formation disappears here, for it is only the land 

 and shallow sea accumulations that thin out. The ocean deposits 

 were still going on, and tlie limestones of the same age extend west- 

 ward and southwestward to the Kocky mountains and ISTew Mexico. 



We learn from these facts that o;eoloo;ical formations of the same 

 age may, at their extremes, be composed of very different materials, 

 and so unlike as to have few or no points of similarity. To us in 

 the east, this period indicates the formation and accumulation of the 

 coal beds, and is really the Gaploniferous ])eriod ', while in the far 

 west and southwest, the term is entirely inappropriate. There it is 

 represented by rocks of limestone, and soft shales or marls, filled 

 with fossils of marine origin and destitute of coal. We have, in the 

 study of this series of strata the most j)ositive evidence of tlie gradual 

 rising of the land on the east, and the gradual evolution of this part 

 of the continent from the bed ot the ocean. 



