18 PROCEEDINGS OF WASHINGTON MEETING. 



stances most susceptible to thermal agency are those formed under ordinary me- 

 teoric conditions, minerals of direct igneous origin being more refractory. 



The outer limit of the aureola, as denned by the production of new minerals 

 undoubtedly due to the metamorphism, is fairly well defined. From there to the 

 granite contact the metamorphism increases progressively, affecting at last all the 

 constituents of the rocks, so that near the granite they are, with special exceptions, 

 completely reconstituted. The changes in character from the outer to the inner 

 limit are so gradual as to render futile any attempt to divide the aureola into suc- 

 cessive distinct zones. The boundary against the granite is always a perfectly sharp 

 one. 



An important problem in connection with thermometamorphism is how far, if 

 at all, are the transformed rocks altered in total chemical composition. It would 

 be rash to give a general answer to this question without much more extensive 

 chemical researches than any yet undertaken ; but there are some facts which throw 

 light on the subject. It is worth remarking, too, that for this purpose igneous rocks 

 present advantages over sedimentary, in virtue of their more homogeneous nature. 

 It is not safe to assume that a mass of slates was originally of one chemical compo- 

 sition throughout, but this difficulty scarcely arises when we can trace a lava flow 

 from beyond the limit of the aureola up to its contact with the intrusive rock. The 

 rocks examined decidedly favor the view that thermal metamorphism is not in 

 general accompanied by any change in bulk analysis. Two exceptions must be 

 recognized. The first consists in the elimination of the volatile constituents of the 

 rocks metamorphosed, viz, water and carbonic acid. The loss of the water, how- 

 ever, does not seem to be complete, hydrous minerals, such as certain micas, often 

 occurring in highly metamorphosed rocks ; while the expulsion of the carbonic acid 

 depends on the presence of silica, free or combined, to take its place, for we find 

 that such expulsion does not operate in the case of a pure limestone. The second 

 exception to the rule consists in the introduction in some cases of certain volatile 

 constituents, such as fluorine and boric acid, and must be referred to the " mineral- 

 izing agents " on which some French geologists have laid stress as necessary con- 

 comitants of an acid intrusion. There is, however, but little trace of these among 

 the Lake district rocks. Tourmaline occurs very sparingly at Shap fell, always 

 close to the granite and always in immediate connection with old joint planes or 

 other fissures, and muscovite is found mostly under similar conditions. Axinite 

 and fluorite are not known. 



In some described cases of thermometamorphism it has been considered that the 

 altered rocks have, in the neighborhood of the contact, received an accession of silica 

 derived from the invading magma. No such process can be verified in the Lake 

 district. Some of the rocks, and especially the rhyolitic lavas and ashes, have un- 

 doubtedly been impregnated with silica, and a similar feature is not uncommon in 

 the acid lavas of northern Wales and other districts. The silica is sometimes seen, in 

 slices, to have replaced feldspar crystals, and the abnormally high silica percentage 

 in some analyses of old rhyolites must be explained by some such secondary action. 

 But the phenomenon in question has clearly no relation to subsequent igneous 

 intrusions, occurring, as it does, often in places far remote from any intruded mass. 

 Whether due to ordinary meteoric weathering or, as seems probable, to solfataric 

 action not long posterior to the cessation of vulcanicity, this silicification cannot be 

 referred to any cause properly described as thermometamorphism. 



