476 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



garnets. These facts led me to argue, in my paper on mica- schist 

 already referred to (Report of British Association, 1856, p. 78), that 

 the alteration of deposits of decomposed felspar into crystallized mica 

 and quartz was not the effect of dry heat and a partial fusion, but was 

 due to highly heated water disseminated through the rock. If so, it 

 is no wonder that ordinary shales have never been converted into 

 mica-schist artificially, by the mere heat of furnaces, since the condi- 

 tions are not those met with in nature — water is absent. 



The mean relative size of the vacuities in the fluid-cavities in the 

 quartz of the slightly metamorphosed schists in Cornwall, at a con- 

 siderable distance from the granite, is '125, which corresponds to a 

 heat of at least 200° C. (392° F.) ; and therefore a considerable 

 thickness of rock must have been raised to a high temperature. If 

 the pressure was great, the temperature must have been still higher ; 

 and on approaching the granite, the relative size of the vacuities 

 indicates nearly as high a, temperature as that at which the granite 

 itself was consolidated, which agrees with the gradual passage from 

 gneiss to granite, and might be used as a strong argument by those 

 who contend that some granites are only thoroughly metamor- 

 phosed stratified rocks. The vacuities in the fluid-cavities in the 

 mica-schist of the southern border of the Highlands of Scotland are 

 relatively so small (v='05) that, if they were formed under no great 

 pressure, they indicate a temperature of only 105° C. (221° F.). It, 

 however, appears to me far more probable that the heat was really as 

 high as in the case of analogous rocks in Cornwall, but the pressure 

 greater. If so, from equation (6) we deduce that the Highland 

 rocks were metamorphosed under a pressure equal to about 23,700 

 feet of rock more than those in Cornwall, or probably when at a 

 much greater depth from the surface ; a result which is confirmed 

 in a most remarkable manner by a comparison of the fluid-cavities in 

 the elvans and granites. These conclusions only apply to when the 

 quartz crystallized : it does not follow that the rock was never heated 

 to a still higher temperature. 



§ 5. Minerals and rocks formed by cooling from a state of 

 igneous fusion. 



The most instructive glass-cavities that I have met with in natural 

 minerals are those in the crystals of clear, transparent felspar con- 

 tained in some of the pitchstone of Arran. Pitchstone, like obsidian 

 and some artificial slags, consists of a glassy base, having no action 

 on polarized light, in which are scattered small crystals that decom- 

 pose it and show colours. The basis of the pitchstone surrounding 

 the crystals of felspar is transparent, and nearly colourless, but 

 contains vast numbers of minute, green, prismatic crystals, probably 

 some variety of pyroxene, often arranged in radiate groups, which 

 impart a deep green colour to the rock. These may be seen to 

 great advantage in thin splinters, but the glass-cavities in the felspar 

 can be studied to far greater advantage in thin sections of the rock. 

 The surfaces of the crystals of felspar are in some cases irregular, 





