METAMORPfflSM. 725 



1. Consolidation. — Ordinary atmospheric or subterranean waters, 

 however prolonged their action, do not necessarily produce solidifica- 

 tion. The soft sandstones of all ages, from the Potsdam to the inco- 

 herent beds of the Quarternary, are evidence on this point. It is 

 probable that deposits have existed to an immense extent in past time, 

 that failed to be consolidated, and consequently were washed away in 

 the course of subsequent changes. 



But while there are many fragile Potsdam sandstones, there are 

 others, as those of eastern New York and Vermont, that have been 

 hardened, through the metamorphic process, into quartzytes and quartz- 

 ose gneisses, and deposits of sand and pebbles of various other ages 

 that are refractory sandstones and grits. That the consolidation took 

 place through the metamorphic process, is often evident from their 

 position within, or on the outskirts of, regions of other metamorphic 

 rocks. In the same way, fragile absorbent argillaceous shales have 

 been hardened into firm non-absorbent slates. 



At the Geyser region of Yellowstone Park, according to F. H. Bradley, the sand-beds 

 of a terrace on Shoshone Lake, over a hundred feet high, have been firmly consolidated, 

 so as to look like quartzyte ; and this was done by the hot siliceous waters, when the 

 waters of the lake stood at a higher level. 



2. Loss of Water or other Vaporizable Ingredients. — The water 

 contained iu the original material of a rock is sometimes wholly, and 

 sometimes but partly, expelled. The volatile ingredients of bituminous 

 coal have been partly or wholly driven off by the process, and anthra- 

 cite and semi-bituminous coal formed (p. 400). 



Carbonic Acid is expelled from carbonate of lime, or limestone, as 

 is well known, in a heated lime-kiln. But, in the metamorphism of 

 limestone, it is retained. It has been shown by experiment that the 

 carbonic acid is not given out, if the material is under heavy pressure. 

 If this be true of carbonic acid, it will be so also of other ingredients 

 less easily expelled. 



3. Change of Color. — Compact limestones are usually of grayish, 

 yellowish, brownish, and blackish colors. From the metamorphic pro- 

 cess they may come out white. The original color, in these limestones, 

 and also argillaceous beds, is often due to carbon, from ancient plants 

 or animal matters ; and, when so, this carbon is removed and the rock 

 blanched by the metamorphism. When oxyd of iron in any form is 

 present, the blanching does not take place unless the oxyd is thrown 

 into some new state of combination, in the crystallizing process. When 

 there is only a partial metamorphism, and the heat is considerable, its 

 presence generally causes a change of color to red. 



4. Obliteration of Fossils. — Rocks that have been subjected to the 

 metamorphic process have usually lost all their original fossils. Where 



