GEOLOGY OF SOIL. 19 



teaches, is from the molten matter of the globe. These 

 have been afterwards, in some cases, removed by water, and 

 in part remodified by heat (5). Referring rocks to their 

 origin, they are divisible into two great classes. 



1st. Those formed by fire. 



2d. Those formed by water. 



11. This division relates both to the origin and distribu- 

 tion. In their origin all rocks are truly igneous, or from fire. 

 In their distribution they are aqueous, or by water. This is 

 the only division necessary to the farmer. It is the division 

 taught and demanded by agricultural geology. 



12. The first class includes all the highly crystalline rocks, 

 granite, gneiss, sienite, greenstone, porphyry ; it includes, 

 also, basalt and lava. 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, pudding-stone, conglomerates, 

 sandstones, slates. When these various substances are 

 examined, a large part of sand is found to be composed 

 essentially of the ingredients 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 dis- 

 tricts, composed almost wholly of some of the chemical 

 constituents of the igneous rocks, united to air. The con- 

 stituents are lime and magnesia ; the air is carbonic acid, 

 forming, by their union, carbonates of lime and magnesia. 

 Marble, limestone, chalk, belong to this formation. These 

 are not to be ranked as original igneous products subse- 

 quently distributed by water. The lime, originally a part of 

 igneous rocks, has been separated and combined with air, by 

 animals or plants, by a living process called secretion. The 



