ORIGIN OF GRANITE. 223 



tent higher up. It is probable that even 300° to 400° Fahr. is sufficient 

 to produce a considerable degree of "change ; or even 200°, if alkali be 

 present. 



Crushing:. — Although simple gravitative pressure is only a condi- 

 tion, and not a cause, of heat, horizontal pressure ivith crushing of the 

 crust, by the conversion of mechanical energy into heat, becomes, as 

 Mallet has shown,* an active source of this agent. Now, in all cases 

 of metamorphism we find ample evidences of such horizontal crushing 

 in the associated foldings and cleavage of the strata. 



Mechanical Metamorphism. — Very recently it has been shown that 

 lateral pressure with crushing and shearing, and sometimes even a kind 

 of flowing of the crushed rock, will produce a schistose structure not 

 only in stratified but even in igneous rocks. This has been called 

 mechanical or dynamical metamorphism. Thus the difficulty of deter- 

 mining the origin of metamorphic rocks becomes still greater. 



Again, percolating water containing silica, even at ordinary tem- 

 perature and pressure, but especially at high temperature under heavy 

 pressure, may often fill up by crystallization the interstices of a sand- 

 rock so as to make a perfect quartzite. f 



Explanation of Associated Phenomena. — This theory readily ex- 

 plains — 1. Why metamorphism is always associated with great thick- 

 ness of strata; 2. Why the oldest rocks are most commonly meta- 

 morphic, since these have usually had the newer rocks piled upon 

 them, and have been subsequently exposed by erosion. The newer 

 rocks are sometimes also metamorphic, but in these cases they are very 

 thick. 3. It also explains the interstratification of metamorphic with 

 unchanged rocks ; since some rocks are more easily affected by heated 

 water than others, and the composition of the included water may be 

 also different, some containing alkali and some not. 4. It also explains 

 its association with foldings of strata and with mountain-chains, as will 

 be more fully explained hereafter. 



If metamorphism is only produced in deeply-buried sediments, then 

 the exposure of such rocks on the surface can only result fron exten- 

 sive erosion. 



Origin of Granite. 



No doubt many granites are the consolidated reservoirs from which 

 eruptions have come ; but there is much reason to believe that most 

 granites are not the result of simple dry fusion, as is usually supposed ; 

 but, on the contrary, only the last term of metamorphism of highly- 

 siliceous sediments, and have not given rise to eruptions at all. Ac- 

 cording to this view, incipient pastiness by heat and water makes 



* Philosophical Transactions, 1873, p. 147. 



f Irving, American Journal, vol. xxv, p. 402, 1883, and vol. xxxi, p. 225, 1886. 



