16 PROCEEDINGS OF WASHINGTON MEETING. 



The following paper was then read : 



THERMOMETAMORPHISM IN IGNEOUS ROCKS. 



-'c 



BY ALFRED MARKER, M. A., F. G. S., OF ST. JOHNS COLLEGE, CAMBRIDGE, ENGLAND. 



The metamorphic effects due to the heat of intruded masses have, from the days 

 of Hutton, received a fair share of attention from geologists, and as regards the 

 phenomena thus induced in various types of sedimentary rocks we are now in 

 possession of a considerable array of facts. Observations on the thermometamor- 

 phism of igneous rocks and of crystalline schists are, however, very few, despite 

 the fact that any such investigation might be expected to throw light on some 

 problems prominent in modern geology. So far as the crystalline schists are con- 

 cerned, indeed, the field is almost unbroken, though such researches as those of 

 Professor G. H. Williams in the Cortland district and of Salomon in the Adamello 

 range have shown it to be a very promising one. In this place I confine myself to 

 some of the facts already ascertained with regard to thermometamorphism in nor- 

 mal igneous rocks. 



The earliest contribution of importance is that of Allport * who drew attention 

 to the uraliti/ation of the augite in the " greenstones " adjacent to granite intru- 

 sions in Cornwall. Lessen f described similar effects in the diabases of the Harz, 

 and more recently Dalmar, j Sauer,§ and Beck || have found diabases converted into 

 actinolite and anthophyllite schists around the syenite of Meissen, etc, in Saxony. 

 All these observations refer to the modifications set up in one family of rocks. 

 Barrois' *[ " diorites," metamorphosed by the Rostrenan granite in Brittany, appear 

 also to have been originally pyroxenic rocks, though the uralitization is not entirely 

 confined to the vicinity of the contact. In the diabases and diorites of the Macon- 

 nais and Beaujolais, metamorphosed by irruptions of microgranulite, Michel Levy ** 

 has described somewhat different phenomena, including the " epigenesis of labra- 

 dorite crystals by the microgranulitic magma." So far the acid irruptives have 

 received no notice, and the same may be said, except for a few remarks by Judd,ff 

 of the whole of the volcanic division. It is with the last-named rocks that I pro- 

 pose to deal in this communication. 



For a study of thermometamorphism in volcanic rocks there can be no more 

 instructive field than the English Lake district. All the central part of that dis- 

 trict is occupied by a great volcanic series of Ordovician age, consisting of both lavas 

 and fragmental accumulations ; and at certain places on the edge of the district 

 these rocks all come within the metamorphosing influence of large igneous intru- 

 sions. 



The lavas belong to three distinct petrographic gi*oups, presenting, despite their 

 geological antiquity, all the characteristics of the volcanic habitus — the fluxional 

 arrangement of their elements, the vesicular structure, the development of porphy- 

 ritic crystals, and (subject to secondary modifications) the isotropic residue. There 



♦Quart. Journ. Geol. Soc, vol. xxxii, 1876, p. 418. 



fErlaut. zur geol. Speeialk. Preuss., Blatt Harzgerode, 1882, pp. 79, etc. 



X Erlaut. zur Speeialk. Konigr. Sachsen, Section Tanneberg, 1889, Blatt 64. 



I Ibid. Section Meissen, 1889, Blatt 48. 



I! Zeits. deuts. geol. Ges., vol. xliii, 1891, p. 257. 



If Ann. Soc. Geol. Nord, vol. xii, 1885, p. 1U2. 



**Bull. Soc. Geol. Fr., ser. 3, vol. xi, 1883, p. 29C. 



ff Quart. Journ. Geol. Soc, vol. xlvi, 1890, p. 370. 



