ORIGIN OF GRANITE. 225 



quartz."* Besides these fluid cavities, however, glass cavities are also 

 found in the quartz and feldspar of granite. These facts point plainly 

 to the agency of both heat and water in the formation of granite. 

 Among the liquids thus inclosed in granite and other metamorphic 

 rocks is often found liquid carbonic acid. This fact shows the great 

 pressure under w T hich solidification of the rock took place. 



Even the temperature at which metamorphic rocks and granite 

 solidified has been approximately determined by Mr. Sorby. The prin- 

 ciple on which this is done is as follows : If crystallization from solu- 

 tion, or solidification in the presence of water, take place at ordinary 

 temperatures, then the fluid cavities will be full ; but if at high tem- 

 peratures, and the mass subsequently cools, then by the contraction of 

 the contained liquid a vacuous space will be formed which will be 

 larger, in proportion to the amount of contraction, and therefore to 

 the temperature of solidification. Knowing, therefore, the relative 

 sizes of the vacuole and the contained water, and the coefficient of ex- 

 pansion of the water and the rock, the temperature at which the cavity 

 would fill (which is the temperature of solidification) may be calculated. 

 Sometimes this temperature may be gotten by actual experiment, i. e., 

 by heating until the cavity fills. By this method Mr. Sorby has deter- 

 mined the temperature of solidification of certain metamorphic rocks 

 of Cornwall as 392° Fahr., and of some granites as 482°, and others 

 only 212°. 



It seems almost certain, therefore, that many granites have not been 

 formed by dry, igneous fusion. Yet that this rock has been in a liquid 

 or pasty condition is perfectly certain from its occurrence in tortuous 

 veins. Therefore it has been rendered pasty by heat in the presence 

 of water under great pressures, such as always exist in deeply-buried 

 strata. The weight of the superincumbent strata, or else pressure by 

 folding and crushing of the strata, has forced it into cracks and great 

 fissures. 



What we have said of granite applies of course to the whole gra- 

 nitic group. Granitic rocks are often only the last term of the metamor- 

 phism of sediments ; granite being produced from the more siliceous 

 sediments, and diabase and gabbro from the more basic impure clays. 

 But we can not stop with this group. It is certain that many if not 

 all the rocks of the Trappean group also may be made by metamor- 

 phism of sediments. Many bedded diorites, dolerites, and felsites, are 

 undoubtedly formed in this way, for the gradations can be distinctly 

 traced into slates. Prof. Dana f has recently recognized this as so cer- 

 tain that he proposes the addition of the prefix meta to these to indi- 



* Sorby, Quarterly Journal of the Geological Society, vol. xiv, pp. 329, 453. 

 f American Journal of Science and Arts, vol. xi, p. 119, February, 18*76. 

 15 . - 



