30-1: MESSRS. A. HARKER AND J. E. MARR ON 



tion of heated waters carrying silica in solution" during a " Solfatara- 

 stage " which may have marked tlie decline of Ordovician vulcauicity 

 in the area. Whether this explanation hold good or not, it is 

 difficult to believe that the alteration observed in some of these 

 rocks could be effected except by the introduction of silica in some 

 manner ; and this addition of silica from without probably explains 

 the high percentages of that substance found in some published 

 analyses of rhyolites. 



Passing on to the thermo-metamorphism of the rhyolites, we find 

 a few points worth recording. Specimens taken north and east of 

 the spot marked " Tunnel," at distances of about 600 or 700 yards 

 from the margin of the granite, have suffered some alteration of the 

 groundmass, which is in places of a microcrystalline texture, showing 

 felspar as well as quartz. This is apparently quite reconstituted, but 

 curving perlitic cracks are still clearly evident throughout the mass, 

 marked out by micaceous films. The rock here encloses small por- 

 phyritic felspars, which are either quite unaltered or partly silicified, 

 as mentioned above. One specimen has numerous vesicles, which 

 are filled b}^ crystallized quartz, partly idiomorphic ; and there is 

 no evidence that this quartz has recrystallized under metamorphic 

 action [801]. 



Near Wasdale Head Farm the rhyolite may be examined close 

 to its junction with the granite, and here more distinct evidences of 

 metamorphism are obtained. 8ome specimens show a microcrys- 

 talline aggregate of recognizable clear felspar and quartz, similar 

 to that noticed in the metamorphosed andesites, and leaving no 

 doubt that the whole has been reconstituted by metamorphic agency 

 [907]. Other examples seem to have been silicified prior to the 

 intrusion of the granite, and the quartz which forms most of their 

 bulk cannot be stated with certainty to have recrystallized during 

 the metamorphism [880, 881]. The same is true of the quartz- 

 veins which traverse some of the slides, the quartz in them often 

 showing partial crystal contours. Besides quartz and felspar, these 

 metamorphosed rhyolites have minute flakes of pleochroic brown 

 mica and some colourless mica giving brilliant interference-colours. 

 Larger flakes of brown mica occur, grouped in a fashion which 

 suggests their derivation from the pale-green decomposition -product 

 seen in some of the non-metamorjAosed rhyolites. A pyrites- 

 mineral, in good crystals, is an occasional constituent of these 

 altered rocks [881]. 



The nodular rhyolites show considerable modifications in specimens 

 taken near the granite. They must have undergone, before meta- 

 morphism, the process so common in these rocks, by which some of 

 the constituents of the nodules became segregated into concentric 

 shells, and these have been variously affected by the metamorphism. 

 We find some bands in the sections consisting almost entirely of 

 moderately coarse quartz-mosaic, with a little mica, either dark or 

 pale, and occasionally crystals of blue tourmaline [907]. These 

 correspond to the flinty shells seen in the nodules of many rhyolites, 

 the silica, which was probably cryptocrystalliue, having been traus- 



