6 GEOLOGY OP SOIL. 



5. A large tract of any extensive country is 

 composed of rocks of a granite texture. This 



.' need? no definition. Such rocks having been ob- 

 »: ;ls8Brv-ed:to underlay all others, in the scale of rocks 

 composing the earth's crust, were called primary. 

 It was supposed that these were first formed. Out 

 of the ruins of these, no matter when or how ruined, 

 other rocks have been made, called secondary. The 

 ruins of the primitive rocks have been transported 

 by water, and then gradually deposited layer upon 

 layer. Under immense pressure, these layers of mud, 

 sand, fine gravel, rolled stones, he, have been, hard- 

 ened into solid rock ; forming sandstones, slates, or 

 even rocks presenting the crystalline structure, or tex- 

 ture of granite, by the action of heat, which the facts 

 of modern geology teach, exists in the interior of our 

 globe. 



6. This central heat is supposed to be the cause 

 of volcanoes, and the primitive rocks themselves, to 

 have been the ejection, under circumstances un- 

 known, of the melted mass of the globe ; ejections, 

 similar in kind, to those of modern lava, but greater 

 in degree. 



7. Intermediate between modern lava, and primi- 

 itive rocks, and actually passing into either, is a 

 large class of ancient volcanic rocks, called, trappe- 



