294 EVOLUTIONAL GEOLOGY. 



which sometimes descend upon our pUmet, arc but portions of its own 

 envelope returning to it. The facts that the average specific gravity 

 of those meteorites W'hich have been seen to fall is not much above 3.2, 

 and that they have passed through a stage of fusion, are consistent 

 with this suggestion. 



SECOND CRITICAL PERIOD, ' ' CONSISTENTIOR STATUS." 



The solidification of the earth probably became completed soon after 

 the birth of the moon. The temperature of its surface at the time of 

 consolidation was about 1,170^ C, and it was therefore still surrounded 

 by its primitive deep atmosphere of steam and other gases. This was 

 the second critical period in the history of the earth, the stage of the 

 "consistentior status," the date of which Lord Kelvin would rather 

 know than that of the Norman Conquest, though he thinks it lies 

 between twenty and forty millions of years ago, probably nearer 

 twenty than forty. 



Now that the crust was solid there was less reason why movements 

 of the atmosphere should be unsteady, and definite regions of high and 

 low pressure might have been established. Under the high-pressure 

 areas the surface of the crust would be depressed; correspondingly, 

 under the low-pressure areas it would be raised; and thus from the 

 first the surface of the solid earth might be dimpled and embossed.^ 



THIRD CRITICAL PERIOD — ORIGIN OF THE OCEANS. 



The cooling of the earth would continuously progress, till the tem- 

 perature of the surface fell to 370° C, when that part of the atmos- 

 phere which consisted of steam would begin to liquef}^; then the 

 dimples on the surface w^ould soon become filled Avith superheated 

 water, and the pools so formed would expand and deepen till they 

 formed the oceans. This is the third critical stage in the history of 

 the earth, dating, according to Professor Joly, from between eighty 

 and ninety millions of years ago. With the growth of the oceans the 

 distinction })etween land and sea arose — in what precise manner we 

 may proceed to inquire. If we revert to the period of the "consisten- 

 tior status,'' when the earth had just solidified, we shall find, according 

 to Lord Kelvin, that the temperature continuously increased from the 

 surface, where it was 1,170° C. , down to a depth of 2.5 miles, where it 

 w^as about 1,130° C, or 260° C. above the fusion point of the matter, 

 forming the crust. That the crust at this depth was not molten but 

 solid is to be explained by the very great pressure to which it was sub- 

 jected — just so much pressure, indeed, as was required to counteract 



^ It would be difficult to discuss with sufficient brevity the probable distribution of 

 these inequalities, but it may be pointed out that the moon is possibly responsible, 

 and that in more ways than one, for much of the existing geographical asyuunetry. 



