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relative size of vacuity (or bubble) to liquid cavity, we should 

 have the two elements from which to calculate the heat; or, if we 

 could form a tolerably accurate estimate of the heat, and make 

 observations on the relative size of vacuity and liquid-cavity, these 

 elements would allow us to form some fairly correct idea of the 

 pressure under which the rock was formed. 



Now, by an elaborate series of investigations, Mr. Sorby has 

 shown it to be very probable that the heat was not much greater 

 than 360° C (680° Fah.), or not more than a dull red heat visible 

 in the dark. We have already seen how the water in many cases 

 contained when hot more mineral salts dissolved in it than could 

 be so contained when cold, hence the small crystals of salt some- 

 times found in the liquid cavities ; but Mr. Sorby has also called 

 attention to the fact that crystals of the mineral called Schorl (a 

 variety of hornblende) are frequently enclosed in the liquid-cavities, 

 and that these would be fused at any temperature greater than a 

 dull red heat; and from this and other facts, he has fixed on the 

 temperature of 360° C (680° Fah.) as the probable extreme at 

 which the quartz of granites was formed. Calculating on the basis 

 of these elements, and having made a large number of observations 

 upon the relative size of the vacuities and liquid cavities in the 

 quartz of the Cornish granites, Mr. Sorby arrived at the conclusion 

 that the mean of the pressure expressed in feet of rock under which 

 the Cornish granites were consolidated was 50,000 feet (this not 

 necessarily the actual depth at which formed). 



A few years ago, being anxious to apply Mr. Sorby's method 

 to the granites and granitoid rocks of this lake country, I examined 

 a number of slices of our granite rocks, and measured nearly 500 

 cases of liquid-cavities, ascertaining the relative size of the vacuity 

 (or bubble) to the liquid cavity. My method of proceeding I will 

 describe directly, but I will at once state that the result arrived at 

 in the case of the Skiddaw granite, for example, was, that its 

 consolidation took place under a pressure of rather more than an 

 equivalent of 50,000 feet of rock ; and that the mean pressure, 

 under which all the principal granitic and granitoid rocks were 

 consolidated, was equivalent to that of 44,000 feet of rock. 



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