A. HAKKER THERMOMETAMORPHISM. 21 



that the total chemical composition of the rocks has remained substantially the 

 same during the metamorphic processes. 



All these metamorphosed rocks, though dating from pre-Carboniferous and prob- 

 ably early Devonian times, present a remarkable freshness of aspect in all their 

 constituent minerals. This is very noticeable in thin slices cut to show the junc- 

 tion of the granite with the volcanics, the feldspars and biotite of the former rock 

 having all the usual signs of weathering decomposition, while the same minerals 

 in the metamorphosed rocks retain unimpaired their pristine clearness. This im- 

 munity from weathering action appears to be a characteristic feature of metamor- 

 phic products, whether formed by thermal or by dynamic agencies, but I have not 

 met with any attempt to frame a general explanation of it. 



So far I have treated the metamorphosed volcanics as a whole, without distinc- 

 tion of the three groups. In the basic and intermediate groups of lavas the changes 

 produced follow very closely the same lines. The original mineralogical differences 

 between the two groups lay chiefly in the relative proportions of their several con- 

 stituents and in the nature of the feldspars, and the metamorphosed representatives 

 do not show any more essential difference. All the foregoing remarks apply to 

 both groups alike, and apply to the fragmental as well as to the fluidal members. 

 In the inner part of the aureola, indeed, the ashes have to be distinguished from 

 the lavas by structural rather than mineralogical characters. 



The acid rocks present a different set of phenomena. In the vicinity of the 

 granite they often consist of an exceedingly fine grained aggregate of clear feldspars 

 and quartz. From this we might suppose that the metamorphism has induced 

 crystallization in rocks originally largely glassy or with special structures not far 

 removed from the vitreous ; but such an inference would not be warranted. In 

 other places we find examples which show little or no evidence of any alteration 

 at all. For instance, I have already mentioned rhyolite fragments in a highly meta- 

 morphosed breccia, which still retain in perfection their micro-spherulitic structure. 

 This is a case in which the visible structure is so intimately hound up with the 

 molecular that one can scarcely imagine a rearrangement of the latter while the 

 former remains uneffaced, and we are almost driven to the conclusion that the 

 fragments are practically in their original condition. This slight susceptibility to 

 thermometamorphism is perhaps to be correlated with the simple chemical compo- 

 sition of our rhyolites, which contain very little iron or lime and no magnesia; so 

 that they have little more than the elements of acid feldspars and quartz. The 

 evident alteration of some of the rocks, on the other hand, may be referable only 

 in part or not at all to thermal metamorphism connected with the intrusion ; for, as 

 already noticed, these rocks have often been affected by earlier changes both physi- 

 cal (devitrification) and chemical (silicification). The ashes associated with the 

 Lake district rhyolites have behaved, as a rule, in a precisely similar manner; hut 

 at some horizons, where a certain amount of magnesia and iron oxides was present, 

 we find to a limited extent the same production of biotite, etc., that characterizes 

 the metamorphosed andesitic ashes. It is evident that in the fragmental volcanic 

 rocks, with their heterogeneous constitution, we cannot expect the chemical group- 

 ing into acid, intermediate, basic, to hold so exactly as in the lavas. It should be 

 noted, as a further point of interest, that the rhyolitic ashes have been more decom- 

 posed than the corresponding lavas prior to metamorphism, and the consequent 

 loss of alkalies has caused andalusite and cyanite to he formed among the metamor- 

 phic products iu place of the usual feldspars, though only to a limited extent. 



