1869.] HUNT — ON VOLCANOES AND EARTHQUAKES. 395 



and by difficultly coercible gases, they are either extravasated 

 among the fissures which form in the overlying strata, or find their 

 way to the surface. The variations in the composition of lavas 

 and their accompanying gases in dilFerent regions, and even from 

 the same vent at different times, are strong confirmations of the 

 truth of this view, to which may be added the fact that all the 

 various types of lava are represented among aqueous sedimentary 

 rocks, which are capable of yielding these lavas by the process of 

 fusion. 



The intervention of water in all lavas, of which it appears to 

 form an integral part, was first insisted upon by Scrope, and is a 

 fact hardly explicable upon any other hypothesis than the one just 

 set forth. Cousidei-ing the conditions of its formation, water 

 would seem to be necessarily absent from the originally fused 

 globe, in which the older school of geologists conceive volcanic 

 rocks to have their source. Scheerer supplemented Scrope's view, 

 by showing that the presence of a few hundredths of water, 

 maintained under pressure at a temperature approaching ignition- 

 would probably suffice to produce a quasi-solution or an igneo- 

 aqueous fusion of most crystalline rocks, and subsequent observa 

 tions of Sorby have demonstrated that the softening and 

 crystallization of many granites and trachytes must have taken 

 place in the presence of water, and at temperatures not above a 

 low red heat. Keeping in view these facts, we can readily 

 understand how the sheet of water-impregnated debris, which, as 

 we have endeavored to show, must have formed the envelope to 

 the solid nucleus, assumed in its lower portion a semi-fluid 

 condition, and constituted a plastic bed on which the stratified 

 sediments repose. These, which are in part modified portions of 

 the disintegrated primitive crust, and in part of chemical origin, 

 by their irregular distribution over difiierent portions of the earth, 

 determine, after a lapse of time, in the regions of their greatest 

 accumulation, volcanic and plutonic phenomena. It now remains 

 to show the observed relations of these phenomena, both in earlier 

 and later times, to great accumulations of sediment. 



If we look at the North American continent, we find along its 

 north-eastern portion evidences of great subsidence, and an 

 accumulation of not less than 40,000 feet of sediment along the 

 line of the Appalachians from the Gulf of St. Lawrence south- 

 wards, during the paleozoic period, "and chiefly, it would appear, 

 during its earlier and later portions. This region is precisely that 



