192 THE PAST AND FUTURE OF GEOLOGY. 



With the great oceans, O, other conditions come into operation which 

 increase the difference, for the cold Arctic waters pass in an nndercur- 

 rent from the poles to the equator with so little loss of heat that near 

 the eqnntor a deep-sea temperature of 35^ Fahr. or even lower exists. 

 Therefore, to the depth of the ocean, we have to add a depth equivalent 

 to the ditference between the mean temperature of the adjacent land 

 and that of the deep waters. In the Arctic zone, the temperature of the 

 land is less than that of the sea, but as we approach the equator the 

 former exceeds that of the latter at depths by as much as 40°, which is 

 equal to a difference in depth of about 2,000 feet. The main channels 

 of the great oceanic troughs in the tropics have a depth of 18,000 feet 

 or more. If we add to this 2,000 feet for the difference of temperature 

 between the surface and the sea-bed, and 4.000 feet for the rise under 

 certain mountain-chains, we shall have a total of 24,000 feet as the 

 approximate difference of level of the isotherm of 1,000° in adjacent 

 continental and oceanic areas.* 



As the position of the other earth-isotherms will in like manner occupy 

 successive planes approximatively parallel with the surface, whether of 

 land or sea-bed, it follows that, if a central molten nucleus exists, it will 

 be divided into areas separated by boundary lines no less important 

 than those formed by the continental areas between the several oceanic 

 areas on the surface ; and, as they are even more inclosed and isolated, 

 their condition with regard to the possible existence of tidal action 

 would approach more to that of an inland sea, such as the Mediterranean, 

 where their influence is scarcely felt. It may be a question also whether 

 the rigidit;y of the earth's crust is not influenced by this mode of structure. 

 It must certainly have affected the permanence of continental and 

 oceanic areas; in the one case by the convexity of the surface favoring 

 elevation, and in the other by the concavity favoring gradual subsid- 

 ence. 



Notwithstanding this,' it may naturally be asked, in view of the more 

 constant slow changes and movements to which in past times the crust 

 of the earth has been subject — and that even up to a period so geolog- 

 ically recent as the elevation of the Alps* and the Andes — how it hap- 

 pens that it is now so quiescent and comparatively immovable. The 

 hypotheses both of Mr. Hopkins and Sir W. Thompson grapple with this 

 difficulty. The former not only considered that the crust was eight 

 hundred to one thousand miles thick, but he also supposed that there 

 were only local and limited bodies of molten matter, the rest of the 

 nucleus having become solid. The latter also concludes, though on other 

 grounds, that the secular refrigeration, combined with the excessive press- 

 ure, has led to a solidification, commencing at the center, of the whole in- 

 terior of our globe;t while, as before mentioned, Mr. Mallet, admitting 



*Tho uuinbcis used are merely approximative. 



tTho Hev. Osmond Fislicr,on the other band, showed in 1873 (Geological Magazine, 

 vol. X, p. 248) that on the supposition of a globe becoming solid throughout at the 



