SORBY — STRUCTURE OF CRYSTALS. 493 



indicates a minimum temperature of 256° C. (493° F.), and a pres- 

 sure equal to about 32,400 feet of rock, or considerably less than 

 some of the el vans. In the Cornish granite I have never found v 

 less than *09, in that from the Ding Dong mine near Penzance, which 

 corresponds to a pressure of 63,600 feet; and the mean of all my 

 observations gives 50,000 feet, or 9700 feet more than the mean for 

 the elvans. This, I think, is a very satisfactory result, since the 

 association of those rocks clearly proves that granite must have been 

 consolidated at a considerably greater depth than elvans. 



The conclusions derived from my examination of the various 

 granites in the neighbourhood of Aberdeen are very striking. In 

 the main mass, at a considerable distance from the stratified rocks, 

 #== about "04 ; and the great difference between vacuities of that size 

 and those in the fluid-cavities in the quartz of the trachyte of Ponza 

 will be seen on comparing fig. 116 with fig. 100, both magnified to 

 the same extent. If the pressure were not taken into account, this 

 would indicate a temperature lower than that of boiling water ; but 

 if the temperature was the same as that at which the quartz of the 

 trachyte of Ponza crystallized, the pressure must have been equal to 

 about 78,000 feet .of rock. If the temperature was higher, the pres- 

 sure must have been still greater. In the exterior part close to the 

 stratified rocks v='07l, which indicates a pressure of 69,000 feet. 

 This is the same as for the porphyries, but 9000 feet less than for the 

 centre of the granite, which appears to me a very reasonable result, 

 since it is extremely probable that the pressure on the outside would 

 be considerably less than in the interior. In some more recent veins 

 of very coarse-grained granite intersecting the other, v = '166, which 

 corresponds to a pressure of only 42,000 feet, as though the condi- 

 tions under which it was consolidated differed materially from the 

 other case, either on account of the elevation of the rocks, or some 

 other physical change. The general mean of all my measurements 

 in the main masses of granite in the south border of the Highlands 

 from Aberdeen to Ben Cruachan indicates a pressure of about 76,000 

 feet, or 7000 feet more than the quartzose porphyry-dykes. The 

 number of cavities is also much less than in those granites formed 

 under a less pressure, as if the crystallization had taken place more 

 slowly, on account of a more gradual cooling, which would probably 

 be the case, if the thickness of the superincumbent rocks was greater. 



Comparing these conclusions and that derived from a comparison 

 of the metamorphic rocks, we have as under : — 



The granites of the Highlands indicate a pressure of 26,000 feet 

 of rock more than those of Cornwall. 



The elvans of the Highlands indicate a pressure of 28,700 feet of 

 rock more than those of Cornwall. 



The metamorphic rocks indicate a pressure of 23,700 feet of rock 

 more than those of Cornwall. 



This remarkable agreement cannot be the result of mere accident, 

 but I think clearly points out that the consolidation of the granites 

 and elvans, and the metamorphosis of the stratified rocks, took place 

 in the Highlands at a very much greater depth than in Cornwall, — 



2 k2 



