22 PROCEEDINGS OF WASHINGTON MEETING. 



Summarily, the chief results as regards the thermometamorphism of volcanic 

 rocks in the English Lake district are as follows : 



1. Basic and intermediate lavas and ashes, especially when affected to any extent 

 by weathering processes, are as readily metamorphosed by heat as are argillaceous 

 sediments. Acid lavas and ashes of simple chemical composition may, however, 

 be very little modified, even by a very high temperature. 



2. Feldspars of various kinds, formed sometimes by the rejuvenation of old 

 feldspars, sometimes by recombinations from other minerals, are universally present 

 in abundance among the new-formed products in the advanced stages of metamor- 

 phism. Andalusite, garnet, and some other aluminous silicates common in meta- 

 morphosed sedimentary rocks are, as a rule, absent. 



3. The characteristic ferromagnesian minerals generated are biotite and green 

 hornblende, augite being exceptional ; and the formation of one or other of the 

 three minerals depends especially upon the percentage of lime in the material 

 metamorphosed. 



4. The only changes in the total composition of the rocks of which we have any 

 evidence in this district are those occasioned by a loss of water and carbonic acid, 

 and rarely and to a limited extent by an accession of hydrofluoric and boric acids. 



I remarked at the outset that investigations into the effects of thermometa- 

 morphism may be expected to throw some light on problems connected with the 

 origin of crystalline schists. The suggestion cannot be properly developed in this 

 place. It may be pointed out, however, that the post-Silurian intrusions of the 

 Lake district, including those of Shap fell and Eskdale, can clearly be referred to 

 the great crust movements which there brought the Silurian period to a close, and 

 which impressed on the whole district its peculiar geological structure. The effects 

 of the lateral thrusts which then operated did not there reach anything like the 

 intensity displayed in the region of the Scottish Highlands, but they furnish, per- 

 haps on that account, an instructive study. Had the mountain-making processes 

 progressed in the Lake district to the same stage as in northern Scotland, we should 

 have dynamic superimposed on thermal metamorphism in the petrographic com- 

 plex formed by the great intrusions, in their minor off-shoots, and in the adjacent 

 altered rocks , but the results of the thermometamorphism would still remain as a 

 factor in the final product* 



The paper was discussed by A. C. Lane, Thomas Macfarlane, of Ottawa, 

 Canada, C. R. Van Hise, and the author. 



After announcements from the chair, Mr. J. F. Kemp, of the Commit- 

 tee on Photographs, announced that the suite of photographs collected 

 by the committee was on exhibition in the Library of the University. 



The Society then took a recess until 2 o'clock p. m. 



*Marr : Quart. Journ. Geo!. Soc, vol. xlvii, 1891, p. 328. 



