CHAPTER XI 



THE SEDIMENTARY ROCKS 



The materials of which the sedimentary rocks are composed 

 were, in the first instance at least, derived from the chemical 

 decay or mechanical abrasion of the igneous rocks, and hence 

 they are often called derivative or secondary. They have been 

 laid down under water (or, in a few instances, on land) and are 

 therefore always stratified and, for the most part, are composed of 

 rounded fragments, seldom crystalline. 



Almost all the materials which we have found in the igneous 

 rocks also occur, in a more or less worn and comminuted condi- 

 tion, in the sedimentary class. However, with the exception of 

 quartz, the great bulk of the sedimentary materials consists of 

 simpler and more stable compounds than the igneous minerals, 

 from the decomposition of which they have been derived. The 

 principal minerals which compose the sedimentary rocks are quartz 

 (Si0 2 ), clay (A1 2 3 , 2 Si0 2 , 2 H 2 0), and the carbonate and sul- 

 phate of lime (CaC0 3 , CaS0 4 ). 



Quartz is a very simple and stable chemical compound, and 

 hence, in the ordinary process of rock decay, it remains unchanged 

 further than being broken up into smaller pieces and rounded by 

 the action of wind or running water. Clay is derived principally 

 from the decay of the felspars, and the lime compounds from the 

 complex silicates containing lime, which are so frequent in the 

 igneous rocks. These rocks also yield the iron oxides which are so 

 widely diffused in the sedimentary class, though comparatively sel- 

 dom in any very great quantity. Very many varieties of rocks 

 are produced by the mixture of the siliceous (quartz), argillaceous 

 (clay), and calcareous (lime) materials in varying proportions. 

 The sorting out of material by water, according to its chemical 



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