66o THE POPULAR SCIENCE MONTHLY. 



ashes — no river, save of molten fire. Now is ending a long history 

 with which the uniformitarian must not reckon — of a time when 

 many compounds now existing were not dissolved but dissociated, 

 for combination under that environment was impossible. Yet 

 there was still law and still order — nay, the present law and order 

 may be said even then to have had a potential existence ; never- 

 theless, to the uniformitarian gnome, had such there been, every 

 new combination of elements would have been a new shock to his 

 faith, a new miracle in the earth's history. But at the times men- 

 tioned above, though oxygen and hydrogen could combine, water 

 could not yet rest upon the ruddy crust of the globe. What does 

 that mean ? This, that assuming the water of the ocean equiva- 

 lent to a spherical shell of the earth's radius and two miles thick, 

 the very lava-stream would consolidate under a pressure of about 

 310 atmospheres, equivalent to nearly 4,000 feet of average rock. 

 Let us pass on to a time, which, according to Sir W. Thomson, 

 would rather quickly arrive, when the surface of the crust had 

 cooled by radiation to its present temperature. Let us merely, for 

 illustration, take a surface temperature of 50° F. (nearly that of 

 London), and assume that the present rise of crust temj^erature is 

 1° F. for every fifty feet of descent, which is rather too rapid. If 

 so, 213° F. is reached at 8,100 feet, and 250° F. at 10,000 feet. 

 Though the latter temperature is far from high, yet we should 

 expect that, under such a pressure, chemical changes would occur 

 with much more facility than at the surface. But many Palaeo- 

 zoic, or even later rock-masses, can now be examined which at a 

 former period of their history have been buried beneath at least 

 10,000 feet of sediment, yet the alteration of their constituents has 

 been small ; only the more unstable minerals have been somewhat 

 modified, the more refractory are unaffected. But for a limited 

 period after the consistentior status, the increase of crust temper- 

 ature in descending would be far more rapid ; when one twenty- 

 fifth of the whole period from that epoch to the present had 

 elapsed — and this is no inconsiderable fraction — the rate of in- 

 crease would be one degree for every ten feet of descent. Sup- 

 pose, for the sake of comparison, the surface temperature as be- 

 fore, the boiling-point of water would be reached at 1,620 feet, and 

 at 10,000 feet, instead of a temperature of 250° F., we should have 

 one of 1,050° F. But, at the latter temperature, many rock-masses 

 would not be perfectly solid. According to Sorby, the steam cavi- 

 ties in the Ponza trachyte must have formed, and thus the rock 

 have been still plastic at so low a temperature as 680° F. At this 

 period, then, the end of the fourth year of the geological century, 

 structural changes in igneous and chemical changes in sediment- 

 ary rocks must have taken place with greater facility than in any 

 much later period in the world's history. Then a temperature of 



