﻿Tol. 66.] SKIDDAW GRANITE AND ITS METAMORPHISM. 139 



presence of a large amount of magnesia, which must have been 

 contained in the finely-divided mica of the slates ; while the original 

 iron must have entered into the biotite. The alkalis appear to 

 have formed colourless micas rather than felspar, which is, indeed, 

 •conspicuous by its absence. In the few cases where felspar was 

 •observed it appears to have always been original (that is, clastic). 

 Although kyanite and sillimanite were sought for diligently, they 

 were not found ; and this fact is significant. The absence of these 

 minerals may be taken as a conclusive proof that the rocks were 

 never subjected to a very high temperature. All the evidence is in 

 favour of the maintenance of a moderate temperature for a long 

 period of time, thus allowing of a very complete recrystallization 

 for a long distance from the margin, although the changes close to 

 the intrusion are not of a very striking character. It thus appears 

 to be demonstrated that the Skiddaw granite was not very hot at 

 the time of its intrusion, and also that it was intruded under a 

 thick cover, so that the heat was maintained for a long time. By 

 this means the molecules were kept in a state of active vibration 

 for a lengthened period, and were thus enabled to enter into new 

 combinations over a very wide area. 



So far as concerns the chemical composition of the metamorphosed 

 rocks, there is no indication whatever of any addition of material 

 from the intrusion, except possibly in the neighbourhood of the 

 Grainsgill greisen, where there is some evidence of pneumatolysis. 

 This question can, however, be decided by general considerations 

 alone, since it is clear that chemical analyses would be utterly 

 useless, owing to the impossibility of ascertaining whether the 

 altered and unaltered rocks were originally similar. The aureole 

 is on a very large scale, and there are no continuous sections of 

 sufficient length to enable any given bed to be traced right through 

 the metamorphosed region. Considerable variations of lithological 

 character occur within a very small thickness of strata, and con- 

 clusions founded on comparison of analyses could have no possible 

 Talue. 



The phenomena here displayed may be very briefly summed up 

 as an example of a moderate degree of thermal metamorphism due 

 to the intrusion of a great mass of granite, at a comparatively low 

 temperature, into a series of rocks of somewhat variable com- 

 position, but on the whole rich in alumina. The most important 

 minerals produced are cordierite, andalusite and its modification 

 chiastolite, biotite and muscovite ; while garnet and staurolite are 

 only found close to the granite. Owing to the great variation in 

 lithological character, it has not been found practicable to divide 

 the aureole into definite zones, but the alteration is gradual and 

 progressive from without inwards. 



In conclusion, I have much pleasure in expressing my great 

 obligation to several friends for help in the somewhat arduous 

 field-work necessary for this investigation, which not seldom in- 

 volved a good deal of physical discomfort, owing to the inclemency 



