350 



PHYSICAL GEOLOGY 



and mica being disintegrated and forming simpler compounds, 

 some of which are dissolved and carried away by the water, white 

 the remainder is left as clay. This clay together with the insoluble 

 quartz is transported by streams and is finally deposited in the 

 ocean, the clay forming mud and the quartz grains, sand. The 

 lime dissolved from the feldspars may be taken up by organisms 

 to form lime ooze or limestone. If these sediments are laid down 

 in a sinking geosyncline (p. 359), they may in time be buried to 

 a depth of several thousand feet. When in the course of their 

 burial they reach the belt of cementation (p. 61), they will be con- 

 solidated into shales and limestones. If the sediments in the syn- 

 cline are subjected to great lateral pressure, heat will be developed 

 which will metamorphose them, changing the clays, sandstones, and 

 limestones to schists, quartzites, and marbles. 



For a discussion of the formation of metamorphic rocks from 

 igneous rocks, see p. 347. 



Weathering of Metamorphic Rocks. — Metamorphic rocks usually 

 resist weathering better than sedimentary ones, because they have 

 been compacted by heat and pressure and have a crystalline texture. 

 Mica schist, for example, is less easily disintegrated than the impure 

 shale from which it was made; hard quartzite than the less compact 

 sandstone; marble, however, may or may not be more resistant to 

 weathering and erosion than the limestone from which it was derived ; 

 the disintegration of slate is hastened by its vertical cleavage, but 

 is hindered by its greater compactness. As a result of prolonged 

 weathering, however, metamorphic sedimentary rocks are reduced, 

 in time, to the same soil which their sedimentary equivalents would 

 have made, schists weathering to clays; quartzites to sands; and 

 marbles to calcareous clays. Because of their greater resistance to 

 weathering, metamorphic rocks are usually associated with the scen- 

 ery of mountains. Quartzite usually resists weathering better than 

 any other rock on account of its small porosity, its insolubility, and 

 its homogeneous composition. Because of the last-named character, 

 it is little affected by changes in daily temperature (p. 31), and 

 it is not disintegrated by the decay of a weaker constituent, as is 

 the case with igneous rocks, such as granite. Quartzite hills are, 

 consequently, among the last to disappear. In regions of meta- 

 morphic rocks, where schist and marbles are involved, streams have 

 usually cut their valleys in the softer and more soluble marbles, while 

 the more resistant schists form hills and mountains. 



