﻿136 MR. R. H.. RASTALL ON THE [May IO/lO, 



to be attributed to the highly siliceous nature of the rocks, which 

 renders them little susceptible to metamorphism. 



A specimen of what appeared to be a highly metamorphosed rock 

 was collected from the middle of the first field west of Swinside, 

 where there is a small boss of well-glaciated rock in place. It is 

 of a deep purplish-brown colour, and shows very well-marked 

 contortions on a small scale, as in Sinen Gill. Under the micro- 

 scope it is seen to be a fine-grained aggregate of grains of quartz, 

 with some felspar (which is probably clastic), together with irregular 

 flakes of the usual brown biotite and subordinate white mica. In 

 places crystals of cordierite are beginning to develop, apparently 

 from quartz and white mica. The crystals are very poorly formed, 

 and are evidently in an embryonic stage. They occur chiefly in 

 bands of finer texture than the rest, where original mica was 

 presumably more abundant. The outlines of the quartz-grains are 

 for the most part irregular, and, where most free from mica, the 

 rock approaches a quartzite in character. 



Another specimen, taken from about a quarter of a mile below 

 the junction of Grainsgill Beck with the Caldew, is on the whole 

 very similar in general character, but the crystals of cordierite 

 show much more perfect development. In this specimen also 

 andalusite occurs in rather small crystals, which are generally 

 irregular, but sometimes idiomorphic ; they frequently show rose- 

 •coloured pleochroism. 



On approaching nearer to the granite the most noticeable change 

 is that the biotite becomes more and more completely converted 

 into a pale-green chloritic mineral, the other constituents remaining 

 the same. Andalusite is present in some specimens, but not in 

 others. The most interesting of all is one taken from the actual 

 contact of the grits with a large dyke of greisen in the bed of 

 Grainsgill Beck. This rock consists solely of quartz, white mica, 

 with a little green chlorite, and abundant garnets : no andalusite 

 or cordierite can be identified. The garnets are very small, 

 colourless in thin section, and isotropic. One slice of this rock 

 -shows the actual junction of the sediment and the igneous rock, 

 ■and in general characters the two are very similar, though easily 

 distinguishable by the naked eye, as well as in the slice. The 

 greisen consists of quartz and muscovite only, and possesses a some- 

 what coarser texture. The garnets and a small amount of chlorite, 

 together with finer texture and a distinct banding, serve to distin- 

 guish the sediment. 'The general character of the rock suggests 

 that the pneumatolytic action which produced the greisen has also 

 had some influence on the surrounding sediment. The meta- 

 morphosed rocks in this part of the district are, on the whole, much 

 paler in colour than elsewhere, apparently because brown mica is 

 -absent. This difference in colour is probably due also to pneuma- 

 tolytic action, since pale or colourless mica is well known to be 

 one of the most characteristic products of gas-action. 



Andalusite-bearing rocks. — In the area just described 



