ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 46T 



formation of a thin crust of stratified rocks ; those rocks being 

 now to be found constituting the so-called primitive gneiss for- 

 mation. 



In accordance with the views given in the second part of this 

 paper, of the nature of the process of solidification at present pro- 

 gressing beneath the earth's crust, we must suppose that during 

 the solidification of the first crust, a contraction of the volume of 

 the originally fluid material took place. This view must be 

 adopted on experimental grounds also. Bischof found, in casting 

 a globe of basalt, twenty-seven inches in diameter, that in the 

 centre of the mass, on cooling, a cavity had formed capable 

 of containing half a pint of water. Further, at the Muld- 

 ner smelting works, near Freiberg, stones are cast of the slag run 

 out of the reverberatory furnaces. They are two feet long, one 

 foot deep and one broad, and when broken after cooling, they are 

 found to contain in the middle irregularly shaped cavities from 

 three to five inches wide, the sides of which are covered with bril- 

 liant microscopic crystals.* From these instances it might be ex- 

 pected, that during the first solidification, a vacuum might, to some 

 extent, have been formed beneath the crust of the earth. With 

 the progress of the consolidation the dimensions of the vacuum must 

 have increased, and the power of the crust to support the enor- 

 mous pressure of the then existing atmosphere must have decreased. 

 We may suppose that ultimately a point was reached, when the 

 crust was unable longer to support the enormous load, and that it 

 then gave way in various places, its fragments sinking down to 

 the fluid interior and floating upon its surface. In this way the 

 first great subsidence of the earth's crust may be reasonably sup- 

 posed to have taken place. The area of the original globe 

 having however decreased during the solidification, it would 

 be impossible for the fragments of the crust to maintain their 

 original horizontal position. Very likely also the still flaid 

 material beneath the crust would protrude itself through be- 

 tween the fragments, thrusting them aside, and limiting still fur- 

 ther the space occupied by the latter. The consequence of this 

 would be, that the fragments would arrange themselves in posi- 

 tions more or less vertical, and, although some of them might still 

 remain horizontal, still highly inclined positions would be the rule. 

 We can even imagine how corrugations of the strata, such as 

 described by Sir William Logan in Canada, and by McCulloch in 



* Leonhard, Huttenerzeugnisse, p, 186. 



