PAL-BONTOLOGIOAL REPORT OF GEOLOGIC At SURVEY. 516 



3rd. THE SHALES OR ROOF SLATES. 



The shales are mostly a compound of the finest particles of matter, 

 deposited in such a way that they are generally laminated in thin 

 sheets, probably a result of periodical influences. If the movement 

 of depression, marked by the formation of the shales, has been as slow 

 as all the appearances lead us to believe it, the water raised above the 

 marshes, was at first nearly of the same depth, and covered the whole 

 field. If we suppose that some essential elements of this water had the 

 power of consolidating themselves, and of imbedding and preserving 

 all the low plants and the leaves falling on the marshes from the trees; 

 if we suppose further, that by breaking the hardened mould, we could 

 still now find the remains of those plants perfectly well preserved in the 

 stones, we cannot but admit that those prints of plants Would give us 

 a pretty exact idea of the Vegetation of the marshes of the coal epoch. 

 It is just what has happened. Whenever, during the formation of 

 the shales, the movement of depression has been so slow that for a 

 length of time the marine water has not invaded the marshes, the de- 

 posited shales contain remains of plants only ; but when the depres* 

 sion has been somewhat more rapid, the deeper water has arrested the 

 vegetation, and the scantily preserved remains of plants are old, much 

 broken, mostly stems, fruits, and pieces of bark of a hard texture, 

 mixed with some shells. 



The presence of the shells in the shales, proves the access of the 

 marine water; it is ordinarily accompanied with some fucoid plants 

 and fishes. The fucoid plants are generally scarce in the coal slates, 

 and the shells, though often represented by an immense number of 

 individuals, are limited to a few species, which differ from those of the 

 limestone, and seem to be of the kind generally living in the contact of 

 the tides with the fresh water of the lakes or rivers. The slow propa- 

 gation of those species lead us to suppose that they were distributed 

 on a vast area, upon the beds where their remains are found. There- 

 fore, if we can admit, that after the formation of each bed of coal, 

 either the plants, or the animals that lived in the water which covered 

 them, were of peculiar species, or at least that some species of plants 

 or shells have either appeared for the first time in each bed of shale, 

 or that identical species have been distributed in each of them in a 

 different proportion, it is evident that the examination of the top or" 

 roof shales of the coal, and the study of their remains, whether plants 



