METAMORPHISM. 707 



different kinds, such as garnet, idocrase, pyroxene, scapolite, mica, spkene, 

 chondrodite, apatite, etc. 



The crystallization which is produced by the process is of all 

 grades, from mere solidification of a bed of shale or sandstone, to 

 the formation of a perfect granite. 



3. Origin of metamorphic changes. 



Promoting cause. — The great promoter of metamorphic changes is 

 subterranean heat, acting in conjunction with moisture, and usually, if 

 not always, under pressure. 



The heat requisite for metamorphism is less than that of fusion ; 

 for the evidence is decisive that although the rocks may be so far 

 softened as to have some degree of plasticity, this is unusual, and 

 for the most part a comparatively low temperature is all that is 

 required. It is probable that the results have generally taken 

 place between 300° and 1200° F. ; but it was heat in slow and pro- 

 longed action, operating through a period that is long according 

 to geological measure. A low temperature acting gradually during 

 an indefinite age — such as Geology proves to have been required 

 for many of the great changes in the earth's history — would pro- 

 duce results that could not be otherwise brought about, even by 

 greater heat. 



The lower limit of temperature is sometimes placed much below 300° F. ; 

 and for consolidation it may be rightly so. But there is definite evidence that 

 it has exceeded this in the majority of cases. In the great faults of the Appa- 

 lachians, 10,000 to 20,000 feet in extent, Lower Silurian limestones are brought 

 up to view, containing their fossils, and not metamorphic. Taking the increase 

 of temperature in the earth's crust at 1° F. for 60 feet of descent, 10,000 feet 

 of depth would give 220° F. as the temperature of the limestone before the 

 faulting, and 20,000 feet would give 390° F. But 1° F. per 60 feet of descent 

 is the present rate, and must be far short of that at the close of the Carboniferous 

 age, when the earth's crust was so easily flexed and metamorphism took place 

 on so grand a scale; and hence the limestone must have been subjected to a 

 heat far above 220° F., if at a depth of 10,000 feet. The length of time, more- 

 over, during which it was thus heated must have been great, as follows from 

 the age of the rocks and the period of the faulting. 



Moisture is essential, because dry rock is a non-conductor of heat 

 (as well shown in the case of a common fire-brick), and also because 

 of its chemical powers when heated. Rocks usually contain some 

 moisture ; and, when moist, heat is conducted readily through them. 



The pressure may have been either that of superincumbent 

 waters or of overlying rocks. A little thickness of the latter would 

 give all the pressure that is in any case essential. 



