44 STUDIES, SCIENTIFIC AND SOCIAL chap. 



feet. Again, it is argued that if the igneous products of 

 volcanoes are derived from one central reservoir there 

 ought to be a great similarity between them, especially 

 between those of the same district. But this is not the 

 case, an example being the Miocene lavas of Hungary and 

 Bohemia, which are of a totally different character, chemi- 

 cally, from each other. But although the molten interior 

 of the globe may be the common source of the heat which 

 causes volcanic eruptions, it by no means follows that the 

 whole, or any large portion, of the matters ejected from 

 volcanoes are derived from it; and it is a remarkable indi- 

 cation of the probable truth of Mr. Fisher's theory, that, 

 as will be shown further on, it entirely removes the two 

 geological difficulties here noticed. At the same time it 

 explains other geological phenomena of a striking character 

 which the theory of solidity altogether fails to account for, 

 as will be now briefly indicated. 



The Argument from Subsidence under Deposition of 

 Sediment. 



It has long been known fco geologists that the series of 

 sedimentary rocks, ancient as well as modern, afford re- 

 peated examples of great piles of strata hundreds, or even 

 thousands of feet thick, which throughout present indica- 

 tions of having been formed in shallow water, and which 

 therefore imply that as fast as one bed was deposited it 

 sank down, and was ready to receive another bed on the 

 top of it. As an example we may refer to the Palaeozoic 

 rocks of the Alleghany Mountains, which are not less than 

 42,000 feet thick ; yet the lowest of these strata, the Pots- 

 dam sandstone, was not deposited in a deep sea, but evi- 

 dently in shallow water near shore, several of the beds 

 exhibiting distinct ripple markings, and the same is the 

 case with the highest strata found there — the carboniferous. 

 On this point Sir Archibald Geikie remarks : — 



" Among the thickest masses of sedimentary rocks — those of the 

 ancient Palaeozoic systems — no features recur more C(3ntinually than 

 the alternation of different sediments, and the recurrence of surfaces 

 covered with well-preserved ripple-marks, trails and burrows of 



