THEORY OF METAMORPHISM. 221 



peutiue may be found in the Tertiary and Cretaceous strata of the 

 Coast Kange of California. But it is also, often, a changed igneous 

 rock containing much olivin (peridotite). 



Theory of Metamorphism. 



There are few subjects more obscure than the cause of metamor- 

 phism, and the conditions under which it occurs. Some important 

 light has been thrown on it, however, recently. For the sake of clear- 

 ness, it will be better to divide metamorphism into two kinds, some- 

 what different in their causes, viz., local and general 



Local Metamorphism is that produced by direct contact with evi- 

 dent sources of intense heat, as when dikes break through stratified 

 rocks. As already seen (p. 210), under these circumstances, impure 

 sandstones are changed into schists, or into gneiss ; clays into slates, or 

 into porcelain jasper ; limestones, into marbles ; and bituminous coal, 

 into coke, or into anthracite. In these cases it is evident that the 

 cause of the change is the intense heat of the incandescent, fused con- 

 tents of the dike at the moment of filling. In such cases of local meta- 

 morphism, the effects usually extend but a few yards from the wall of 

 the dike. 



General Metamorphism. — But in many cases Ave can not trace the 

 change to any evident source of intense heat. Eocks thousands of feet 

 in thickness, and covering hundreds of thousands of square miles, are 

 universally changed. The principal agents of this general metamor- 

 phism seem to be heat, water, alkali, pressure. 



That heat is a necessary agent is sufficiently evident from the gen- 

 eral similarity of the results to local metamorphism. But that the heat 

 was not intense, and therefore not sufficient of itself to produce the 

 effects, is also quite certain. For (a) metamorphic rocks are often 

 found interstratified with unchanged rocks.* Intense heat would have 

 affected them all alike, or nearly alike, (b.) Many minerals are found 

 in metamorphic rocks which will not stand intense heat. As an example, 

 carbon has been found in contact with magnetic iron-ore, although it 

 is known that this contact can not exist, even at the temperature of red- 

 heat, without reduction of the iron-ore. (c.) The effect of simple dry 

 heat, as shown in cases of local metamorphism, does not extend many 

 yards, (d.) Water-cavities are found abundantly in metamorphic rocks. 

 This will be more fully explained farther on. 



Water. — Heat combined with water seems to be the true agent. 

 Recent experiments of Daubree, Senarmont, and others, prove that 

 water at 400° C. (= 752° Fahr.) reduces to a pasty condition nearly all 

 ordinary rocks ; moreover, that at this temperature crystals of quartz, 



* American Journal of Science, vol. xxi, p. 327, 18S1. 



