﻿118 ME. E. H. EASTALL ON THE [May I9IO, 



of the floor of the Oaldew valley in its widest expansion, due 

 north of the summit of Saddleback and east of Skiddaw. The major 

 part of this area is covered with peat, but the boundaries of the 

 granite can be traced in the tributary streams, although it is mostly 

 hidden in the intervening spaces. 



The form of the exposure, as thus defined, indicates that what is 

 Ihere seen is the more or less horizontal upper surface of a large 

 intrusion, which is most probably in the nature of a laccolith. 

 This laccolith undoubtedly underlies the whole of the metamorphosed 

 area at a small depth — thus accounting for the intensity of the 

 .alteration produced in the rocks, and for the great size of the aureole, 

 in comparison with the smallness of the visible exposures of igneous 

 rock. 



The third and most interesting exposure occurs at the junction 

 of Grainsgill with the Caldew valley, and it comprises more than 

 one variety of rock. It consists in part of normal granite, similar 

 to that of the other exposures, and in part of a quartz-mica rock or 

 greisen. This latter has been exhaustively described by Harker. 1 

 The Grainsgill intrusion is very irregular in form, and it must be 

 regarded as an apophysis rather than as an exposure of the laccolith. 

 Both the igneous intrusion and the surrounding rocks are traversed 

 by large and numerous quartz-veins, often containing peculiar 

 minerals and especially compounds of tungsten (wolfram and 

 scheelite), which are now being somewhat extensively worked at 

 the Grainsgill Mine. There is here clear evidence of the occurrence 

 of pneumatolysis of a rather peculiar type. 



Petrography of the Granite. 



To the naked eye the granite shows little variation ; when fresh 

 it is white or grey, and is seen to consist of quartz, felspar, and 

 biotite, with generally some muscovite in addition. The texture 

 is often rather coarse, and commonly large phenocrysts of white 

 felspar, measuring up to 2 inches in length, are scattered here and 

 there throughout the mass ; varieties wholly free from porphyritic 

 felspars are not common. 



Specimens from the three visible exposures are very similar, and 

 all three show much the same range of variation, so that it is hardly 

 necessary to describe each in detail. The freshest examples are to 

 be found in the River Caldew, above Grainsgill, and this may be 

 taken as a type for the northernmost intrusion, when in its normal 

 unaltered condition, without conversion to greisen. 



A typical example of this rock is somewhat coarse in texture and 

 of fairly even grain, without much tendency to porphyritic structure. 

 It consists of quartz, various felspars, muscovite, and biotite, with 

 few accessory minerals. Quartz is abundant, and quite normal in 

 character : it is in most cases the last mineral to crystallize. The 



1 Quart. Journ. Geol. Soc. vol. li (1895) p. 139. 



