ASSOCIATED METAMORPHTC ROCKS OF THE LAKE-DISTRICT. 587 



Downward pressure Surplus pressure 



I I 



30,000. 22,000. 



Downward pressure Surplus pressure 



! I 



27,000. 



22,000. 



The conditions of solidification must evidently have heen somewhat 

 different in the two cases to produce rocks differing from one another 

 in character as these do ; but the general agreement of the results in 

 these and other examples drawn from microscopic investigation and 

 geological field-evidence is satisfactory. 



General Remarks. — The mean value of v for all the above rocks 

 is -158, which corresponds to a pressure of 44,000 in feet of rock. 

 Sorby's mean for the Cornish granites is 50,000 ; and if the Lake- 

 district granites be considered apart from the other rocks mentioned 

 in the table on p. 575, the mean of pressure in feet of rock for 

 them alone becomes 47,000, a very near approximation to that of the 

 Cornish granites. 



Arranging the rocks we have been considering in order of the 

 values of j), and placing in a parallel column the minimum tempera- 

 ture at which consolidation could have taken place in each case, if 

 p — 0, the following result appears: — 



Hunt's 



Fox's 



results, 



mean, 



1 in 85. 



1 in 49. 



238° C. 



406°C. 



271 



463 



284 



486 



310 



537 



310 



537 



330 



565 



350 



599 



Ennerdale and But term ere Syenite 

 Quartz Felsite of St. John's Vale . . . 



Eskdale Granite 



Shap Granite 



Quartz Felsite dyke of Armboth and 



Helvellyn 



Quartz Felsite of Fairy Crag, Crag 



Bridge 



Skiddaw Granite 



Tempera- 

 ture, 

 ifp=0. 



267°C. 



248 



244 



228 



228 



218 



208 



Pressure 



at 360° 



C. 



35000 

 40000 

 42000 

 46000 



46000 



49000 

 52000 



Upon the left are placed other two columns, giving the temperature 

 at depths of the pressure-value, according to Fox's and Hunt's 

 estimates of increase on descending. In the foregoing parts of this 

 paper I have preferred to use Fox's mean of 1 in 49 as probably 

 representing the truth more nearly in early Palaeozoic times than it 

 may now ; and on this supposition we find that all the temperatures 

 at depths varying from 35,000 to 52,000 feet are above 360° C. 

 (680° F.), and range from 400° C. (752° F.) to 600° C. (1112° F.). 



If these results be compared with the tables given by Mr. Sorby 

 on pages 495 and 496 of his memoir, the general agreement is found 

 to be satisfactory. 



Of course it can by no means be taken as certain that the quartz 



of all granites finally became solid at a temperature of 360° C. ; and 



some of the discrepancies in the results brought forward may well be 



due to this difference in the temperature of solidification*. These 



* See note on p. 577, 



