Geology.^On the Crust of the Earth, 293 



at the bottom of the geological series, which have obtained the 

 name of granite^ from the granular form of their constituent mi- 

 nerals. The class of rocks to which granite belongs, has, for dis- 

 tinction's sake, been called primitive : this designation was given 

 in the infancy of geology. It was conceived because no rocks 

 had been discovered beneath granite, that it was the most an- 

 cient mineral portion of the earth, and hence it got the hypo- 

 thetical name of primitive. To steer as clear as possible of hy- 

 pothesis, we shall not use that term, but speak of those rocks as 

 Primary, in relation to their position in the geological series in 

 the ascending order; that is, counting from the granite to the 

 arable soil at the surface, or the diluvium, as this also has been 

 hypothetically called, though perhaps with better cause. Of the 

 depths to which this granite extends we know nothing; it is true, 

 we know from inspection, that in many situations the volcanic 

 lavas come through the granite, and of course may infer that 

 they exist in a state of fusion beneath the granite, or — which is 

 a reasonable conclusion derived from the affinity of their consti- 

 tuent parts— that they are granite in a state of igneous fusion, 

 and that consequently there must be vast cavities in the planet,, 

 inferior to the crust. 



The existence of volcanic action through every part of the 

 known world, either by the eruptions of active volcanos, or by 

 earthquakes, is an assurance that there must be vast cavities in 

 the globe, where igneous action is fiercely at work, and of which 

 these volcanoes are the safety valves. Of their extent, some 

 opinion can be formed from the great distances at which parti- 

 cular earthquakes have been felt. That of Lisbon, in 1755, not 

 only affected the lakes and springs in every part of Europe, but 

 was sensibly felt in North America. That of NeW Madrid, in 

 1811, shook the valley of the Mississippi for several hundred 

 miles. Such disturbances are to be considered as the effect of 

 the resistance which the solid parts of the crust of the earth op- 

 pose to the expansive power striving in those profound cavities. 

 We at length apply this force to many phenomena of our science, 

 and thus comprehend what would otherwise be incomprehensi- 

 ble : it is thus we come to understand how the tops of the high- 

 est mountains, and the bottoms of the lowest valleys, are formed 

 of the same primary rocks ; for when we observe some of. the 

 stratified beds which lie much higher up in the series than the gra- 



