COMPOSITION AND ARRANGEMENT: 131 



tervals thin seams of coal, like those of Gaspe* in Canada, 

 which seemed to have resulted from the growth and drift of 

 terrestrial vegetation. Wherever it occurs its sedimentary 

 character is sufficiently apparent, and though frequently 

 intersected by dykes and eruptive masses of basalt and fel- 

 stone, its stratified arrangement is never wholly obliterated. 

 The interstratifications of volcanic ash and igneous over- 

 flows observable in the Silurian system, and so frequent in 

 the Carboniferous, are rarely witnessed in connection with 

 the Old Eed Sandstone, as if the period, in the north of 

 Europe at least, had been one of comparative internal 

 quiescence. The system occupies considerable areas in 

 Europe, Asia, Africa, and both Americas, and is chiefly of 

 marine formation, though in some districts the total absence 

 of shells and corals would lead to the inference of fresh- 

 water conditions.* 



We have said that the system occupies extensive areas 

 both in the Old and New World, and as no two rivers carry 

 down the same kind of debris, and no two seas receive ex- 

 actly the same kind of sediments, there will be considerable 

 diversity in the character of its rocks that is, in colour, 

 composition, and arrangement. Not only so, but as the 

 system is often of great thickness (12,000 feet or more), 

 there had been oscillations of the crust or new distributions 

 of sea and land during the long period of its deposition, 

 and thus its lower, middle, and upper portions differ even 

 in the same region, and sometimes lie unconformably upon 

 each other. It is for this reason that geologists speak of 



* If the Old Red Sandstone of Scotland be of marine origin, it seems 

 inexplicable why no sea-shell, coral, or other zoophyte should have yet 

 been detected in any of its strata. Numerous as its fishes and Crustacea 

 undoubtedly are, and gigantic as some of them may appear, they may 

 have been inhabitants of estuaries or fresh- water seas ; and though the 

 general belief leans towards oceanic conditions, we are still without un- 

 mistakable proofs to support it. 



