126 THE BOOK OF NATURE STUDY 



The activity of the earth's interior has therefore a second 

 influence on the crust. It not only lifts sediments and incorporates 

 them with the land areas, but it directly contributes new crystalline 

 rock to the land masses, and this is of course, like the sedimentary 

 rocks, capable of being disintegrated and transported to lower 

 levels by denuding agencies. A corollary from the present heated 

 state of the inside of the earth is that it is now cooling down from 

 some former intensely heated condition. If this were the case, fluid 

 water would have been an impossibility at that stage of the earth's 

 history. Nothing but igneous rocks could be formed at that time, 

 and the only new additions of solid material must have come from 

 the cooling of the heated interior matter. Thus all sediments 

 must be ultimately traceable to crystalline igneous material. 



It has been found that the requisite material for clastic rocks 

 can all be obtained from one or other of the many types of crystalline 

 rock. Thus granite would provide broken grains of crystalline 

 quartz to make sand-grains. Any of the felspars would yield grit 

 fragments if fresh, or if decomposed, silicate of alumina for clay 

 rocks. Ingredients dissolved from felspathic rocks include salts of 

 potash, soda, or lime, the last eventually redeposited as limestone. 



The vicissitudes passed through by rocks before, during, and 

 after elevation to form land masses have their effect in inducing 

 important changes in them, and in giving them the new characters 

 which they now exhibit in the form of rocks. 



First of these comes consolidation. Sediments on their 

 formation are loaded with water, but the weight of the accumula- 

 tion squeezes out much of this, drives the particles together making 

 them to some extent interlock, and thus converts the mass into 

 a fairly compact substance. Clays pass into shales, and sands 

 into soft sandstones. During elevation the lateral pressure pro- 

 duces similar effects in a more important degree, and to this the 

 making of compact slaty rocks out of clays is to be attributed. 

 But a more important consolidating agency is the deposition 

 between the grains of a sediment of crystalline cement from its 

 solution in water. Carbonate of lime, silica, carbonate and oxide 

 of iron, are thus deposited. The grains are now held tightly 

 together, and the mass is converted into a thoroughly solidified 

 rock which will stand weight and pressure, and will only undergo 



