72 ALTERED ROCKS NEAR BASTOGNE. [Feb. IQOI, 



Hot springs were potent factors in the metamorphism of rocks. 

 He had studied their effects in the Himalayas, and found 

 that the rocks in their vicinity — especially basalts — had been 

 intensely metamorphosed. It was sometimes difficult, however, 

 to distinguish between the results of contact-metamorphism and 

 the results produced by hot aqueous agents circulating through 

 rocks underground. Geologists were aware that the quartz of 

 granite abounds in water- vesicles, and water was usually supposed 

 to contribute by its presence to the fluidity of molten granite. An 

 appreciable part of the contact-action of granite is due to the 

 superheated w^ater, or steam, emanating from the fluid magma; 

 and the resulting metamorphism is sometimes not distinguishable 

 from the action of water caught up in, or percolating through, 

 sedimentary beds, and brought within the influence of underground 

 heat and pressure. Some minerals, such as mica and hornblende, 

 are known to be the products of contact-action and also of the 

 simple percolation of water below the surface of the earth. 



Prof. Watts referred to the brief account, published in the 

 Annual Report of the Geological Survey, of the metamorphic areas 

 of the Isle of Man. These areas showed what appeared to be 

 typical examples of contact-metamorphism associated with masses 

 of granite and other intrusive rocks ; but it has been found by 

 Mr. Lamplugh that the metamorphism had no relation to the 

 intrusive masses, indeed it was visible in the intrusive masses 

 themselves. Minerals similar to those described by the Authoress 

 occurred in the Isle of Man. The hypothesis of the action of 

 mineral springs was one which would have to be carefully considered 

 in the Isle of Man as well as at Bastogne. 



The President and Prof. Sollas also spoke. 



Prof. BoNNEY, replying on behalf of the Authoress, said that as 

 the Isle-of-Man rocks, mentioned by Prof. Watts, were not yet 

 really described, she could not be expected to refer to them. The 

 Wicklow rocks he had seen, under the kind guidance of Prof. Sollas, 

 and had examined with the microscope ; but they were markedly 

 different in more than one respect from those in the Bastogne area. 

 Here the chief peculiarity was, that remarkably well-formed garnets 

 occurred in a comparatively unaltered matrix. In ordinary contact- 

 metamorphism (examples of which, as the Fellows had just seen, did 

 occur in the Bastogne area) the garnets were seldom well-shaped, 

 and only appeared when the matrix was greatly changed. So far as 

 his own experience went, the Bastogne rock was almost unique, and 

 its mode of occurrence was difficult to explain, either as the result of 

 contact- or of dynamic metamorphism. As to the last, undoubtedly 

 work produced heat ; but the question was, how much ? If the 

 crushing was sudden, there might be a considerable rise of 

 temperature, but then the vicinity should show great disturbance ; 

 if it were slow, there would be no effective rise of temperature. 



