730 DYNAMICAL GEOLOGY. 



Herschel brought forward the argument that, since there is an increase of tempera- 

 ture for every sixty feet of descent in the earth's crust, if strata should accumulate 

 over a region in the sea to a depth of 10,000 feet, the heat would rise accordinglj- into 

 the stratified mass; and, as the same temperature as before would exist at a depth of 

 sixty feet, there would be accordingly in the lower part of the mass the same elevated 

 temperature that existed 10,000 feet below the former surface — this being a means of 

 raising heat from below without disturbance, and a degree of heat that in some circum- 

 stances might be sufficient for metamorphism. But, if metamorphism had actually 

 taken place in this way, we should expect to find sections showing horizontal or slightly- 

 disturbed metamorphic beds, and a gradual transition through a series of such beds to 

 an absence of metamorphism ; but this has nowhere been observed. The great Appa- 

 lachian faults and the Nova Scotia coal-series are direct testimony against the theory. 

 (Am. Jour. Sci., III. vi. 13.) 



4. Metamorphism in some cases obliterates differences in rocks, and 

 in others intensifies them, (a.) Differences obliterated. — A coarse 

 conglomerate and a granitic sandstone associated with it may have 

 come from the wear of the same granitic rocks, and hence may, by 

 metamorphism, be made into a uniform bed of granite. . It is possi- 

 ble, also, that the same identical granite might come from an argilla- 

 ceous deposit or shale, since such a shale may consist of the same gra- 

 nitic ingredients in a finer state of division. 



In the volcanic regions of South America and Mexico, the partial metamorphism of 

 volcanic tufas (both sandstones and conglomerates) has produced red rocks spotted with 

 crystals of feldspar, which are so like red porphyry that they have been mistaken for it 

 by good geologists. They often show their pebbles only on worn surfaces. 



(b.) Differences intensified. — On the other hand, layers of argilla- 

 ceous sandstone, differing but little in color or texture, may be pro- 

 foundly different after metamorphism, one becoming in the change a 

 whitish gneiss, another a dark-gray mica schist, another hornblende 

 rock or schist, another chlorite slate, etc., these differences depending 

 on the presence or absence of oxyd of iron, feldspar, and one or two 

 other ingredients which do not make much impression upon the ap- 

 pearance of the unaltered material, and on the amount of heat and 

 moisture concerned. 



A purely siliceous sandstone, and one a little argillaceous, are looked upon as essen- 

 tially the same rock : and, in the study of sedimentary formations, the difference would 

 hardly attract attention. But, after subjection to the metamorphic process, the purely 

 siliceous sandstone comes out quartzyte, while the argillaceous may be either gneiss 

 or quartzytic mica schist, or hydromica slate, or chlorite slate, or hornblende schist, 

 rocks very unlike quartzyte. Grave errors are often committed in consequence of not 

 appreciating this class of facts. 



4. Metamorphism of Metamorphic Recks. 



Metamorphic rocks are not proof against further metamorphism. 



Among the Archaean rocks of northern New York (in Fowler, De Kalb, Edwards, 

 Russel, Gouverneur, Canton, and Hermon, St. Lawrence County), there are extensive 

 beds of a kind of soapstone (called rensselaerite) which has in places the cleavage of 



