158 



Prof. J. Prestwicli. 



In mostly all of existing volcanoes, there is clear evidence of the 

 access of sea water in the presence not only of the chlorides and other 

 products of its decomposition in the emanations from the lava, but also 

 in many cases of sea salt itself. Not, however, that the presence of sea 

 water is necessary or is always present ; for the vast stores of under- 

 ground water in the Andes, and the great distance from and height 

 above the sea of so many of the South American volcanoes, render it 

 probable that they are in the main dependent upon inland water 

 supplies, and this is confirmed by the researches of Boussingault, who 

 was unable to detect in the fumaroles he examined any traces of 

 chlorides. 



But if water only plays the secondary though important part I 

 have assigned to it, to what are we to attribute the motive power 

 which causes the extravasation of the lava? This involves questions 

 connected with the rigidity and thickness of the earth's crust, that we 

 will proceed to consider in their geological bearings. 



§ 6. Thickness of the Earth's Crust from the Geological Standpoint. 



Geologists and physicists still hold from their different standpoints 

 divergent views respecting the thickness to be assigned to the crust of 

 the earth. Although the present stability of the earth's surface 

 renders it evident that the hypothesis of a thin crust resting on a 

 nucleus altogether molten and fluid is untenable, it is equally difficult 

 to reconcile certain geological facts with the hypothesis of a globe 

 solid throughout, or even of a very thick crust. 



Nor have the phenomena of the tides yet been determined with 

 sufficient accuracy to settle definitely the moot question whether the 

 rigidity of the crust is perfect, or whether it yields to some very 

 small extent to the deformation that might be caused by slight internal 

 tides. Therefore, while on this ground alone, even if the data were 

 not otherwise insufficient, there are, on the other hand, certain geo- 

 logical phenomena, volcanic phenomena among the rest, which are not 

 only incompatible with an entirely solid globe, but which would seem 

 to be explicable only on the hypothesis of a thin crust and a slowly 

 yielding substratum. Of course it is also open to consideration, 

 whether a crust and substratum of this nature would not, under 

 certain conditions, offer sufficient resistance to produce a quasi rigidity 

 such as would accord with existing physical conditions. 



The phenomena on which on geological grounds I should chiefly 

 rely in proof of such a substratum, and of a crust of no great thick- 

 ness, are — 



1st. The flexibility of the crust as exhibited (a) in the uplift of 

 mountain chains, and (b) in the elevation of continental 

 areas. 



