4 8 



SOILS. 



crystalline, that is, composed of more or less distinct (large or 

 minute) crystals of one or several of the minerals mentioned 

 above. 



3. Eruptive rocks, ejected in the molten state from vol- 

 canoes or fissures; crystalline or not, according to slow or 

 rapid cooling. 



Sedimentary Rocks. Sedimentary rocks are forming to-day 

 by deposition from either sea or fresh water, precisely as they 

 were in the most remote geological times; the oldest clearly 

 sedimentary rocks being sometimes undistinguishable in their 

 nature and composition from the very latest immediately pre- 

 ceding our present time. They may for the purposes of the 

 present work be simply classified as follows : 



1. Limestones, formed in comparatively shallow seas, or 

 fresh water basins, from the calcareous shells or skeletons of 

 various organisms. 



2. Sandstones, and conglomerates (sometimes called pud- 

 ding-stones) formed from the debris of pre-existing rocks dis- 

 integrated by the agencies described above, (chap. 1-2), re- 

 cemented by means of solutions of one or several substances, 

 such as silex, carbonate of lime, ferric hydrate and others. 

 Loose sands and gravels are the initial stages of such rock for- 

 mation as well as the results of their disintegration. 



3. Clays, Claystoncs and Clay shales, consisting of clay sub- 

 stance with more or less sand, and soft or hard according to 

 the nature of the waters or solutions that may have acted upon 

 them, with or without the aid of heat. These rocks can only 

 be formed in comparatively quiet or " back " waters, since 

 clay would not ordinarily be deposited in moving water. 



Mctamorphic Rocks. The effects of subterranean heat or 

 metamorphism upon the sedimentary rocks may be roughly 

 stated as follows: 



Limestones are transformed into marbles of various degrees 

 of purity, according to the nature of the original rocks. 



Sandstones when cemented by silex are transformed into 

 quartzite, of greater or less purity according to the nature of 

 the " sand " entering into its composition. When cemented 

 by materials other than quartz, these also will be segregated 

 in the form of various minerals in the body of the rock. 



