8 GEOLOGY OF SOIL. 



distribution. In their origin all rocks are truly igne- 

 ous or by fire. In their distribution they are aque- 

 ous or by water. This is the only division neces- 

 sary to the farmer. It is the division taught and de- 

 manded by Agricultural Geology. 



12. The first class includes all the highly crystal- 

 line rocks, granite, gneiss, sienite, greenstone, porphy- 

 ry,* basalt, lava, volcanic sand. The products of 

 volcanoes, whether ancient or modern, agricultural 

 geology places in the same class, including thus all 

 that portion which forms the largest part of the 

 earth's surface. 



13. The second class includes sand, clay, gravel, 

 rounded and rolled stones of all sizes, puddingstone, 

 conglomerates, sandstones, slates. When these va- 

 rious substances are examined, a large part of sand 

 is found to be composed essentially of the ingre- 

 dients of the igneous rocks. This is true also, of 

 sandstone, slate, of conglomerates, of bowlders. 



14. There is a large deposit, or formation in some 

 districts, composed almost wholly of one of the 

 chemical constituents of the igneous rocks, united to 

 air. The constituent is lime, the air is carbonic 

 acid, forming by their union carbonate of lirne. 

 Marble, limestone, chalk, belong to this formation. 

 These are not to be ranked as original igneous pro- 



