ATMOSPHERIC AGENCIES. 15 



1. Sandstone is a rock made up of grains of sand cemented 

 into a mass, sometimes by lime carbonate, sometimes by 

 silica. Under the slow action of atmospheric water the 

 cement is dissolved, and the rock crumbles into sand, 

 moistened with a solution of lime carbonate, if this be the 

 cement. 2. Granite and gneiss and many other igneous 

 and metamorphic rocks, such as are found on the eastern 

 slope of the Appalachian Chain everywhere, are an aggre- 

 gation of four minerals, viz., quartz, feldspar, mica, 

 and hornblende. In coarse granite these can be easily 

 seen with the naked eye. The bluish glassy specks are 

 quartz ; the opaque white, or rose-color, are feldspar ; 

 the glistening scales are mica ; and the black spots, horn- 

 blende.* The whole rock may be regarded as grains of 

 quartz, mica, and hornblende cemented into a mass by 

 feldspar. Now, quartz is not at all, and mica very slightly, 

 affected by atmospheric water ; but the feldspar and 

 hornblende are slowly changed into clay, which, in the 

 case of hornblende, is red, from the presence of iron. 

 Thus, the whole rock rots down to a clay soil, usually 

 red, in which are disseminated grains of quartz and 

 scales of mica, the whole moistened with water, contain- 

 ing in solution a little potash derived from the feldspar. 

 This is the commonest of all soils. 3. Slates and shales 

 are clays hardened into rock by some cement such as 

 lime or silica. When the cement is dissolved the rock 

 crumbles into a clay soil. 4. A pure limestone like mar- 

 ble makes no soil because it is all soluble, but most lime- 

 stones are mixed with clay or sand. When the lime is 

 dissolved the result is a limy clay or limy sand. 



Mechanical Action of Air ; Frosts. The soil-for- 

 mation, above explained, is a chemical process, but, in 

 cold climates and mountain-regions, atmospheric water 

 acts also mechanically and very powerfully in rock-dis- 



* These minerals ought to be shown the pupil, both separately and 

 as aggregated in a specimen of coarse granite. 



