Dishtrhance of the Crust of the Earth. 3 



vious to the formation of glaciers, in a condition different from 

 its present one, and such as would have admitted of some 

 geological change adequate to produce such an alteration of 

 that condition as might have originated glaciers ; and, second, 

 for means of a gradual restoration to the first condition of 

 temperature, such as might bring about the changes that have 

 since effected a diminution of the elevation and extent of the 

 glaciers. I will consider, therefore, — 



I. What the condition of the crust of the earth probably 

 was before the most recent disturbance occurred. 



II. The probable cause which brought about that disturb- 

 ance. 



III. The effects which were likely to follow. 



1. It has been very generally supposed that the elevation 

 of the primitive rocks into their present position has been 

 caused by the invasion of granite and other matter which we 

 find filling the veins which traverse them. This appears to 

 have been a mistaken notion. We see lofty precipices, the 

 faces of which exhibit veins of granite traversing gneiss, and 

 the granite flush with the rock. Now, had the granite been, 

 as is supposed, in a fluid state at the moment when the strata 

 were broken and pushed upwards, so as to form precipices 

 clear of the rest of the rock, the fluid matter must have run 

 out of the spaces occupied by the veins. It is, then, an un- 

 avoidable inference that the granite was solid at the period 

 of elevation ; and that it is a mistake to affirm the force which 

 drove the granite into the fissures of the rock to have been 

 the same, in point of time, that gave to the masses their pre- 

 sent disrupted appearance. The same observations apply to 

 trap. Before precipices displaying trap-veins were elevated, 

 the trap had invaded the rock and become solid. As we find 

 trap cutting granite, we ascribe this to a period more recent 

 than that when the granite was produced. We also find trap- 

 veins cutting older trap, and these also must have existed be- 

 fore the rocks were brought into their present position, so as 

 to give to the earth its uneven and broken aspect. As we do 

 not find granite traversing trap, but observe numerous in- 

 stances of trap cutting granite, it is obvious that the invasion 



