Ill OUR MOLTEN GLOBE 51 



the globe at the sea-level, it is evident that there must be 

 some denser matter under the oceans to make up for the 

 much less density of the water, which is at least three 

 miles deep on the average. A very refined mathematical 

 investigation shoAvs that this can only be brought about 

 by the sub-oceanic crust being both thinner and denser 

 than under the continents, the denser portion being the 

 upper layer. This distribution of matter may, it is sup- 

 posed, be due to extensive outflows of heavy basalt over 

 the original depressions forming the ocean floors, at some 

 early period of their history. 



The physical constitution of the liquid matter forming 

 the substratum is the next point to be considered, and is 

 one of the highest importance, since it is evidently what 

 determines both volcanic action and a large portion of the 

 disturbances to which the crust is subject. Many geolo- 

 gists are of opinion that the phenomena of volcanic action 

 can only be explained on the supposition that the molten 

 matter forming the interior of the globe holds in solution 

 enormous quantities of water- vapour and other gases; and 

 there is ample evidence that melted lavas and slags do 

 contain such gases, which they give out on becoming 

 solid. Thus Mr. Scrope, in his great work on Volcanoes, 

 says : — 



' ' There unquestionably exists within and below volcanic vents, 

 a body of lava of unknown dimensions, permanently liquid at an 

 intense temperature, and continually traversed by successive volumes 

 of some aeriform fluid, which escape from its surface — thus present- 

 ing all the appearance of a liquid in constant ebullition." 



And again : — 



"If any doubt should suggest itself, whether this fluid is 

 actually generated within the lava, or only rises through it, having 

 its origin in some other manner, it must be dispelled by the evidence 

 afibrded in the extremely vesicular or cellular structure of very 

 many erupted lavas, not merely near the surface, but throughout 

 the mass, showing that the aeriform fluid in these cases certainly 

 developed itself interstitially in every part." 



Professor Judd, in his volume on the same subject, 

 shows that the presence of these gases in lava is in accord- 

 ance with Henry's law, that liquids ate able to absorb 



