ASSOCIATED METAMOKPHIC ROCKS OE THE LAKE-DISTRICT. 577 



I have already said that 38,000 feet is very likely the greatest 

 depth at which the Skiddaw Granite could have been consolidated ; 

 hut, in all probability, the depth was considerably less. Even if we 

 suppose the Upper Silurians to have maintained the same thickness 

 northwards which they have in the Kendal district, it is doubtful 

 whether we can include more than 4000 feet of Skiddaw Slate in 

 the total thickness of strata overlying the granite at the period of 

 its formation. This being the case, the estimated thickness would be 

 reduced to 30,000 feet ; and the difference between that and the 

 calculated pressure thus becomes 22,000 feet, to be expended in the 

 elevation and contortion of the overlying rocks. 



It is curious to contrast these results with those of the increase of 

 temperature on descending through the earth's crust. "We are sup- 

 posing that the Skiddaw Granite was consolidated at a depth of 

 30,000 feet arid at a temperature of 360° C. (680° P.). Now, if we 

 take the rate of increase of heat with depth as given by Mr. B. "W". 

 Pox*, viz. 1° for every 49 feet, we shall find that the temperature 

 of 360° C. (680° P.) would be attained at a depth of 30,900 feet. 

 On the other hand, Mr. Hunt's estimate, as quoted by Mr. Sorby in 

 his paper, would give this temperature at the much greater depth of 

 53,000 feet, agreeing very nearly with the calculated pressure of 

 rock in feet given as the result of microscopic examination. It is 

 hard to say which is the mean rate of increase most to be relied on; 

 and it is quite possible that, even if Mr. Hunt's estimate be nearest 

 the truth at the present epoch, that given by Mr. Pox might repre- 

 sent the conditions at an early Palaeozoic period, before cooling of 

 the earth's crust had gone so far as it now has. Thus, to sum up in 

 the case of the Skiddaw Granite, we may say it is probable, from 

 geological evidence, that this granite was consolidated at a depth of 

 about 30,000 feet. Other evidence gives a temperature of 350° C. 

 (662° P.) for this depth beneath the surface ; microscopic evidence 

 gives a pressure in rock of 52,000 feet at the temperature of 360° C. ; 

 this calculated pressure exceeds the estimated depth by 22,000 feet, 

 which excess may have been applied to the work of elevation and 

 contortion of the overlying 30,000 feet of rock. 



We now proceed to see how such arguments are borne out by 

 other cases in the same district. 



* Brit. Assoc. Report for 1857, p. 91. [An important paper by Prof. Mohr 

 has just appeared (Neues Jahrbuch, 1875, 4th part, p. 371) on the cause of 

 earth-temperature. A boring near Berlin gave a temperature of 38 '5° R. at a 

 depth of 4042 ft. The rate of increase of temperature per 100 ft. was found 

 to diminish in a constant ratio with the depth, so that at 5170 ft. it would be 0. 



(1° 5° \ 



j^ E. instead of y^) tne conn! aril 



temperature is reached at 13500 ft. This, says Mohr, refutes the doctrine of 

 plu tonic fire, and leads to the inference that the internal temperature is the 

 result of actions going on in the outer crust. It is needless to remark upon tho 

 bearing of this on Mr. Mallet's theory of vulcanicity. At the same time it 

 seems to render it more probable than ever that granite has not been formed 

 at a high temperature, though under conditions of great pressure and mois- 

 ture.— Oct. 1875]. 



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