43^ : GEOLOGY, 



of the earthy basic element (the calcium, magnesium, and iron oxides) 

 into the calcareous, magnesian, and iron deposits, while (5) a large part 

 of the alkahne basic remainder (the sodium, and potassium oxides) is 

 dissolved and held in the sea-water. Physically, the great features are 

 (1) the disaggregation of the antecedent rock, and (2) the separation from 

 one another of products which are physically unlike, that is, the coarser 

 from the finer, and the heavier from the lighter, and (3) the aggrega- 

 tion of these diverse materials in more or less distinct beds. It is to be 

 noted that while the rearrangement of the sediments is made on the basis 

 of their physical characters, it results in chemical differentiation as well, 

 for the products of rock decay, which are physically diverse, are often 

 chemically diverse as well. The physical assortment and the stratifica- 

 tion are to be looked upon as a step in the direction of a simpler group- 

 ing of the material. On the whole, the process is descensional in 

 character. 



THE REASCENSIONAL PROCESS. 



Running hand in hand with this descensional process, there has 

 always been a reascensional process by which the coherence, the crystal- 

 lization, and in some measure the complex composition of the rocks 

 are restored. This is partially due to external mechanical agencies, 

 but chiefly to internal chemical and molecular forces. 



Two general phases of this reconstructional work are recognized. 

 The first, simplest and most universal, is that by which the incoherent 

 materials produced by the descensional processes, i.e., the muds, sands, 

 and clastic materials generally, are hardened into firm, coherent shales, 

 sandstones, and limestones, and incidentally more or less changed in 

 composition and molecular arrangement. The second is that by which 

 more profound changes of induration and of composition are wrought, 

 bringing the rock back to a state resembling its original crystalline 

 character. This is known as metamorphism. Often, however, it is 

 but an extension and intensification of the more common processes of 

 the first class. Metamorphism is essentially reconstruction. 



Induration under ordinary pressures and temperatures. — All kinds of 

 loose fragmental material, whether soils, earths, clays, sands, gravels, 

 volcanic ashes, cinders, or other forms of clastic or pyroclastic material, 

 may become hardened into firm rock either by pressure, or by cementa- 

 tion, or by both. Pressure and cementation commonly act together and 



