62 GEOLOGY OF AGRICULTURE. 



tered over the earth's surface, in the form of boulders 

 and pebbles, by what has been termed the drift agency. 

 Trap-rock, of which there are two kinds, hasoJt and 

 gremstone, seems to have resulted from the volcanic ao 

 tion of a later, but still very ancient period. IMountain 

 ranges of this are also found in various places, as the 

 Holyoke range in Hampshire county. Lava is the re- 

 sult of still more recent volcanic action, including that 

 of volcanoes now in existence. The granite, trap-rock, 

 and lava, which appear on or near the surface, are 

 therefore to be considered as having come from deep 

 in the earth. They have been forced up, as lava still 

 is, by volcanic action. Their presence above the aque- 

 ous rocks, in such vast quantities, indicates an immense 

 amount of the same materials below them. Next below 

 the aqueous rocks, is supposed to be that vast amount 

 of granite, of which the portions existing on the sur- 

 face of the earth, as thrown up by ancient volcanoes, 

 are but mere specimens. Next below the granite is 

 supposed to lie the trap-rock, from which the less an- 

 cient volcanoes were supplied with the material which 

 they belched forth. Below the trap, geologists be- 

 lieve, is the molten lava, which existing volcanoes are 

 now throwing out. It has been ascertained bej^ond a 

 doubt, that as we descend into the earth the tem- 

 perature becomes warmer, at a rate that would Sring 

 it to the melting point of rock, at something like forty 

 miles from the surface. It has therefore been inferred 

 that the solid crust of the earth cannot be more than 

 40 or 50 miles in thickness. This crust, or shell, is sup- 

 posed to be made up, first, above the lava of trap-rock, 

 then granite • then the acueous rocks, the primary, the 



