MATERIALS OF THE EARTH 57 



Metamorphism by heat, pressure, and water. The mechanical 

 results produced by pressure are always attended by the evolution 

 of heat, and the heat and the pressure, in the presence of water, 

 which is almost always in the rocks, greatly facilitate chemical 

 changes. The result is that the mineral matter of the crushed 

 and heated rock is often re-combined and re-crystallized. Under 

 pressure, the new crystals arrange themselves so that their longest 

 diameters are at right angles to the greatest pressure, and this 

 orientation of the new crystals, like the orientation of other par- 

 ticles, helps to develop schistosity. 



It is to be observed that two distinct processes may be involved 

 in the making of schists. The one is the metamorphism of clastic 

 rocks into crystalline schists, which may be regarded as an up- 

 building process; the other is the mashing down of massive crystal- 

 line rocks into schists, which may be regarded as a descensional 

 process. As a rule, neither process goes on alone in the develop- 

 ment of schists. In both, there is more or less solution and re- 

 arrangement of the molecules, and in both there is probably some- 

 thing of crushing. 



The kind of schist produced depends on the constitution of 

 the rock metamorphosed. Thus basic rocks give rise to basic 

 schists, and acidic rocks to acidic schists. It is obvious that 

 ordinary shales cannot usually become basic schists, because in 

 the production of the muds from which the shales are made, the 

 bases were generally removed; but when shales are highly cal- 

 careous and magnesian, as when they grade toward limestone 

 and dolomite, they may become basic schists (say hornblendic 

 schists) by metamorphism. It is obvious that limestone and sand- 

 stone must retain largely their distinct composition. 



Completion of the rock cycle. The crystallizing processes of 

 metamorphism are fundamentally similar to the processes by 

 which rocks crystallize from lavas; but in metamorphism, the 

 work is done chiefly by the aid of an aqueous solution , while in 

 the solidification of lavas the crystallization is from a mutual 

 solution of the constituents in one another,, where water was but 

 an incident. 



