86 



On the Constitution and Construction of the 



to which it conducts us, are in harmony with facts derived 

 from other sources. 



" The results arrived at," says Mr Hopkins, " have an im- 

 portant bearing on our physical theories of volcanic forces, 

 and the mode in which they act. Many speculations respect- 

 ing actual volcanoes have rested on the hypothesis of a direct 

 communication, by means of the volcanic vent, between the 

 surface and the fluid nucleus beneath, assuming the fluidity to 

 commence at a depth little, if at all, greater than that at 

 which the temperature would suffice, under merely the atmo- 

 spheric pressure, to fuse the matter of the earth's crust. 

 When it is proved, however, that that crust must be several 

 hundred miles in thickness, the hypothesis of this direct com- 

 munication is placed much too far beyond the bounds of pro- 

 bability, to be for an instant admitted as the basis of theo- 

 retical speculations. We are necessarily led, therefore, to the 

 conclusion that the fluid matter of actual volcanoes exists in 

 subterranean reservoirs of limited extent, forming subterra- 

 nean lakes, and not a subterranean ocean. Such, also, we 

 conclude, from the present thickness of the earth's crust, must 

 have been the case for enormous periods of time ; and, conse- 

 quently, there is a very high degree of probability that the 

 same was true at the epochs of all the great elevations which 

 we recognise, with the exception, perhaps, of the earliest.** 



Let figure 2 represent a section of a portion of the globe 

 to illustrate the hypothesis. 



