726 DYNAMICAL GEOLOGY. 



the metamorphism is partial, the fossils may in part remain, only ob- 

 scured. A Devonian coral limestone, near Lake Memphremagog, and 

 at Littleton, New Hampshire, contains some nearly perfect corals ; but 

 most of them are much flattened and indefinite in outline, and others 

 are only patches of white crystalline carbonate of lime in a bluish- 

 gray limestone rock, which is itself hardly at all crystallized. 



The crystallization, in some cases, involves no change of composi- 

 tion. This is the fact with most limestone ; the ordinary compact 

 rock may be simply changed by the process to a crystalline-granular 

 condition, and bleached in color. Argillaceous shales are changed to 

 mica schists, and argillaceous sandstones to gneiss or granite. But, 

 while the alteration in texture is very great, the clays or argillaceous 

 deposits very often, as stated on page 649, contain the minerals of the 

 latter, even in the requisite proportions, so that metamorphism is only 

 a change in crystalline condition. 



But in other cases the constitution of the original bed is altered, 

 new mineral species being formed. Even in the case of limestone, the 

 impurities are turned into crystalline minerals of different kinds, such 

 as garnet, idocrase, pyroxene, scapolite, mica, spJiene, 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. — One 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 was required. It is 

 probable that the results have generally taken place between 300° and 

 1,200° F. ; but it was heat in slow and prolonged action, operating 

 through a period that is long even 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 produce results that coiild not be other- 

 wise brought about, even by greater heat. 



The lower limit of temperature is sometimes placed much below 300° F. ; and for con- 

 solidation 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 Appalachians, 10,000 feet, or more, 

 in extent, Lower Silurian limestones are brought up to view, containing their fossils, 



