Geology 



planes of deposition, but the case of slate is very different. 

 Originally a deposit of clay, slate has been produced by 

 the effect of great pressure, which has resulted in a 

 structure called "cleavage" that renders it possible to 

 split the rock into thin sheets. The direction of the 

 cleavage has no direct relation to that of the bedding 

 planes. 



Slightly coarser kinds of mud give rise to such 

 materials as mud-stones, sandy shales, and fine flag- 

 stones, while from sandy deposits we eventually get 

 coarse flagstones, sandstones, and grits. 



When river gravels or the shingle beaches of a coast- 

 line become consolidated they produce the rock known 

 as conglomerate, of which there are many types differing 

 from one another both in the nature, size, and form of 

 the pebbles, and in the amount and structure of the finer- 

 grained matrix in which they are embedded. 



Limestones are, for the most part, the products of 

 animal and vegetable life. The substance matter of 

 limestone is calcium carbonate, and this is, under certain 

 conditions, soluble in water. It is present in a state 

 of solution in most natural waters, including those of the 

 sea and of lakes. Many of the animals and plants which 

 inhabit these waters have the power of absorbing the 

 calcium carbonate and secreting it in a solid condition, 

 as shells or other structures, and it is by the accumulation 

 of whole and fragmentary shells, corals, calcareous sea- 

 weeds, and the like that limestone masses are slowly 

 built up on the ocean floors and in other suitable 

 situations. 



Coal and lignite, or brown coal, are examples of the 

 formation of rocks by the accumulation and consolidation 



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