286 Scientific Geology. 



with its organic remains, is, that all of them are of vegetable origin. 

 Whether all of them grew upon dry land is not so certain ; if, as I 

 have suggested, a Fucoides occur in this rock. The beds of an- 

 thracite, however, prove that this rock was formed, in part at least, 

 on a surface elevated above the waters. Yet we must call in the aid 

 of powerful currents to account for the accumulation of rounded nod- 

 ules, which are so abundant and so large in some varieties of this 

 rock : and then the inclined position of the strata proves that they 

 have been elevated since their deposition. Indeed, it seems difficult 

 to explain all the phenomena in this graywacke, or any other forma- 

 tion containing coal, with some marine relics, without supposing at 

 least one or two elevations above, and depressions below the waters, 

 during the period of its deposition. And if we admit that various 

 portions of the earth's crust have from time to time been elevated, it 

 is easy to conceive that other portions must have been depressed. 



6. ARGILLACEOUS SLATE. 



This is one of those rocks whose mineral character is clear ; it 

 being composed of schistose clay, more or less indurated. This 

 character, however, applies to the shale of the secondary rocks, which 

 few geologists now include under argillaceous slate. It also applies 

 to some of the slaty varieties of graywacke : and Dr. Macculloch in- 

 cludes under argillaceous slate tho whole of the graywacke formation, 

 as well as that argillaceous slate which is associated with rocks older 

 than graywacke. But De la Beche limits the term to this last named 

 slate, excluding the graywacke, which he puts into a separate group. 

 I prefer his arrangement upon the whole ; though in adopting it I 

 find myself perplexed to determine whether the argillaceous slate 

 around Boston, belongs to his " Inferior stratified or Non-fossiliferous 

 Rocks," or is a member of the graywacke group ; where, for the 

 sake of convenience, I have described it. I am inclined to believe 

 that it should be separated from the graywacke ; though with my 

 present information, I cannot say exactly where the dividing line 

 should run. But the fact that no organic remains have been hitherto 

 discovered in this slate, and the occurrence of its rolled nodules in 

 some of the conglomerates of the graywacke, as well as its geological 

 position, render it probable that it is considerably older than most 

 varieties of graywacke. 



In respect to the other patches of argillaceous slate exhibited on 



