I02 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



the granite in the glen between that mountain and Blencathra. 

 Of these rocks my lamented friend Clifton Ward has given iis a 

 good description accompanied by some valuable analyses *. The 

 ground-mass of the well-known chiastolite slate, which is the first 

 stage of the alteration of the ordinary Skiddaw slate, does not 

 materially differ from that of an ordinary phyllite or satiny slate. 

 It exhibits a banded structure, produced mainly, it seems, by the 

 greater or less quantity of a very dark, probably carbonaceous, mineral, 

 though there are other slight differences, such as in the size of the 

 micaceous constituents, which, however, may possibly be due to the 

 same cause. This ground-mass is composed of minute films of 

 mica, probably sericite, with numerous mineral granules, which I 

 do not attempt to name, but which are doubtless mostly alumina 

 silicates. With these are mixed, in variable quantities, tiny granules 

 of quartz (whether original or secondary I cannot say, probably 

 both), and the black constituent — carbonaceous matter and iron- 

 oxide. The mineral-banding, according to Mr. Ward, corresponds 

 with the planes of cleavage. The only mineral of notable size and 

 well-defined outline is the chiastolite ; and I cannot help thinking 

 that this belongs to a late stage in the alteration of the rock. The 

 crystals are well defined externally, bear no marks of compression, 

 and are fairly free from enclosures. Much of the carbonaceous 

 matter seems to have been expelled during the growth of the 

 crystals and forms a black external border. The filmy ground-mass 

 is pushed aside, although the distortion of the layers does not pro- 

 ceed very far. Except, however, for these chiastolites the rock, as a 

 whole, is still in an early stage of metamorphism. As we approach 

 the intrusive granite, the rock becomes much indurated and 

 evidently is more highly changed ; it fractures readily across the 

 cleavage-planes, so as to form the well-known " whetstone slate." 

 On examining microscopic sections of these rocks, we find that the 

 process of change appears to have been as follows : — The rock was 

 at first a fine-grained mud (for the analysis shows that there cannot 

 have been much free quartz). It must have been brought (probably 

 after being cleaved) to a condition which allowed fairly free motion 

 among its molecular constituents. Certain of these have combined to 

 form well-defined crystals of a mica of the biotite group ; others, a 

 mica of the paragonite group; the black constituents have to a great 

 extent disappeared ; very numerous microlithic prisms, probably a 

 subsilicate of alumina, have been formed. Numerous rather elongated 

 elliptical spaces may be observed comparatively free from these prisms, 

 the micas, and the black spots ; these I think we may safely recog- 

 nize as ill-developed crystals of andalusite, and there are a few more 

 definitely crystalhne (chiastolite?). These last two, as in the 

 former case of the chiastolite, have to some extent extruded the more 



* After more than one examination of the Lake district, I am obliged to say- 

 that I cannot jSnd the slightest ground for the theoretical views of my friend 

 as to the more important intrusive masses of igneous rocks being portions of 

 the sedimentary rocks melted dovm, I believe that the cause of this error (as 

 I hold it to be) was an over-estimate of the amount of metamorphism in the 

 Borrowdale series. 



