101. 



woods, and affect drainage and water circulation. The Appalachian slates 

 when slanting let lands slip, and thus give riss to slips. 



b) Metamorphic rocks. 



Ketamorphic rocks undergo changes from their original struc- 

 ture of igneous or sedimentary rocks thru the agencies of heat, pres- 

 sure, -^nd water. Practically all of the mnterial in metamorphosis be- 

 comes crystalline. Here \?e find quart zite; granites weather slowly and 

 deeply, producing a deep and good soil. Grthoclase yields potash, as al- 

 so doon syenite. They may tend to heavy clay l^nds. 



Diorites and diabases give more potash than the last named rocks, 

 the diabase being the "better of the two, tho not very extensive; they 

 are good for soils, vica schists and gneisses are modifications of gra- 

 nite "/ith much niicr. layered. They consist of mic^ and qjjactz, with lit- 

 tle feldspar; they produce lean and coarse soils. 



c) Eruptive rocks. 



Eruptive rocks may be either acid or basic. Silica is found 

 here. They usually produce good soils, tho sometimes stony in character, 

 They may be considered as occurring in three forma: 



1) Massive forms; basalt is common here, particularly in 

 columnar form. 



2) Broken lava, sometimes in cracked layers, in Northern Cali- 

 fornia. 



3) Spongy form or pumice stone. Large areas in the northwest, 

 nr> in the Cascades, are covered with pumice sands. 



H) Methods of Soil Formation, or. How Soils are tfade. 



There are three general phases in the formation of soils: 

 Physical changes: weathering, disintegration . 

 Chemical changes: decomposition, recomposition. 

 Transportation. 

 Physically 



li.^Rock breaks up due either to change of temperature or to 

 freezing of >^ter in crevices. Chemical action by water dissolves it 

 and transfers it. (v/ith plenty of time and plenty of water almost any- 

 thing will dissolve, even the most indissoluble of substances, to some 

 extent). Leeching of materials is important. 



Rock weathers faster in a hot and desert country,; also by freez- 

 ing; by being broken mechanically; and by being ground by glaciers, wa- 

 ter and winds. Huch rock is worn and dissolved by water, especially suci: 

 as contains acids (C02) Part of the rock is carried in solution; this 

 is very important in connection with the formation of fresh soil. 



Plants assist the physical work; they protect arid hold the rock 

 surface, and their roots hold the soil. 



b > . Chemical action is one of decomposition and recompositiori. 

 This prcTceas is aided by temperature in some parts of the country, and 

 by humidity. Egyptian monuments which lasted for thousands of years in 

 the dry climate of 3gypt were brou^t to America, and they undoubtedly 

 will not last in our humid climate; they have already shown signs of 

 giving way. ater and carbon dioxide are again the main agents in che- 

 mical decomposition of rAcks, together with other acids, soluble salts 

 and alkalies. Plants aid physically, but also add chemicpl substances 



