HEAT METAMOKPHISM. 311 



such a source. In outcrops of an Archaean granite, the feldspar and mica are 

 usually as perfect as when made in Archsean time, excepting a thin layer of 

 surface alteration. So in many of the outcrops of trap, the pyroxene 

 and labradorite are still unchanged pyroxene and labradorite ; and this, 

 though millions of years have intervened since the outflow ; and millions 

 of years of uniformity are sufficient to prove stability. The thin layer 

 of surface alteration indicates the depth of permeation, and to this depth 

 there is alteration, but not metamorphism. Buried in subterranean waters, 

 the conditions would be the same except that even surface alteration would 

 be prevented; for a sandstone that will fall to pieces when exposed to 

 the air will make durable underwater abutments. A trap ledge that 

 decays to a depth of two or three feet, when it is above the tide-level, will 

 remain solid and wholly unaltered below low tide. Pyrite and other 

 iron-bearing minerals oxidize, and help on the decay in the outer layer where 

 it is exposed to the air ; but below this they remain unchanged. White 

 marble, although a more porous rock than most others, usually retains its 

 whiteness perfect through the body of the rock, its pyrite and other imbedded 

 minerals losing nothing in their luster or composition. 



2. Heat above the ordinary tem^ierature usually necessary. — Lyell attrib- 

 uted metamorphism to the heat of the earth's interior. The rocks bore 

 evidence, in the position of the beds, of upturnings and of great pressure ; 

 and those which were left deepest as a consequence of the movements became 

 crystalline or metamorphic. They were hence also called by him Hypogene 

 rocks. Effects from dynamical forces were here recognized, but the heat 

 was statical heat. 



This continued to be the theory of geologists until 1868, when Henry 

 Wurtz, of New Jersey, in the American Journal of Milling, announced the 

 principle that metamorphism was due to heat derived from the friction at- 

 tending the upturning of the rocks, that is, to heat of dynamical origin. 

 In the editions of this work since that date this theory of regional 

 metamorphism, through heat of a dynamical source, has been adopted. But 

 it has also been recognized that heat of a dynamical source has been more or 

 less supplemented by heat from the earth's interior, that is, by statical heat. 

 At the same time statical heat has been referred to as also the source of local 

 metamorphism. It should be observed here that it is the heat that is 

 dynamic, not the metamorphism ; for the metamorphism is the same whatever 

 the source of the heat, whether dynamical or statical, except in some minor 

 points due to pressure, as explained beyond. 



3. The presence of moisture. — All rocks are permeated by moisture, and 

 this permeating moisture is sufficient for all metamorphic results. The 

 amount ordinarily present is stated on page 205. If 2-67 per cent, which is 

 less than the average, the amount would correspond to two quarts of water 

 for each cubic foot of rock. At one per cent it would be one pound, and, 

 therefore, one pint of water to 100 pounds or two thirds of a cubic foot of 

 rock ; and, since a pint contains 29 cubic inches of water, this amount w^ould 



