A. HARKER — THERMOMETAMORPHISM. 17 



are 1 1) basic lavas (hypersthene-basalts) with about 51 per cent of silica, characterized 

 by basic feldspars, hypersthene, and iron ores, but without olivine ; (2) intermediate 

 lavas (pyroxene-andesites) with 59 per cent of silica, some containing a monoclinic, 

 some a rhombic pyroxene, and some both ; (3) acid lavas (rhyolites) with 75 per 

 cent of silica, showing various phases of the glassy type, with feeble porphyritic 

 development and a strong tendency to microspherulitic and macrospherulitic 

 structures. These acid lavas, in which the ferromagnesian minerals are almost 

 completely wanting, bear a close resemblance to certain American Tertiary rhyo- 

 lites, such as those described by Mr. Whitman Cross from Custer county, Colorado ; 

 and the other types of Lake district lavas are not difficult to parallel among the 

 newer volcanic rocks of the United States and central Europe. 



The fragmental igneous rocks of the English Lake district, varying from fine 

 submarine ashes to coarser breccias and agglomerates, are associated with each of 

 the three groups of lavas. Those belonging to the acid group are chemically similar 

 to the rhyolites themselves, and are not always easily distinguished from them in 

 the field. The ashes and breccias associated with the intermediate and basic groups 

 are often more acid than the lavas, owing to the inclusion of numerous rhyolite 

 fragments. All the fragmental rocks, though of subaqueous formation, are in 

 general of purely volcanic origin ; but some of the rhyolitic ashes and breccias in 

 the upper part of the series (which passes up into the Coniston limestone group) 

 contain a variable admixture of foreign material, both detrital and calcareous. 



On the western side of the district the volcanic rocks are in contact with extensive 

 intruded masses — the granophyre of Buttermere and Ennerdale and the granite 

 (often granophyric also) of Wastdale and Eskdale, — and extreme metamorphism 

 has been set up. The same phenomena are presented in equal variety and with 

 greater clearness in the neighborhood of the granite of Shap fell on the eastern 

 edge of the district. Here, too, ordinary sediments, calcareous, argillaceous and 

 arenaceous, come within the same metamorphosing influence, and afford a standard 

 of comparison for the effects produced in the volcanic rocks. It may be remarked j 

 also, that the district offers an admirable field for the study of dynamometamor- 

 phism in the same rocks and for comparison of the two lines of modification which, 

 as here developed, give rise to widely different phenomena. The Shap fell tract 

 in particular has been carefully examined by Mr. J. E. Marr and myself. Details 

 of field-work and chemical and microscopic study would be out of place here, and 

 have been recently published elsewhere ; * but the results give occasion for some 

 remarks bearing on thermometamorphisrn as a whole, and thus possessing a general 

 interest. 



The metamorphic aureola of Shap fell extends for about three-quarters of a mile 

 from the visible granite outcrop, and this distance is nearly the same whether we 

 take it in the volcanics or in any of the sedimentary groups. In this connection, 

 however, it should be noted that the volcanic rocks had undergone considerable 

 alteration by meteoric agencies prior to the intrusion of the granite, which took 

 place in post-Silurian times. Such evidence as we have goes to show that fresh 

 volcanic rocks would be less readily affected by thermometamorphisin. At the 

 outer edge of the aureola it is only the decomposition products of the intermediate 

 and basic rocks that have been transformed; similarly in the sedimentaries it is 

 the calcite, carbonaceous matter, etc. The general rule seems to be that the sub- 



* Quart. Journ. Geol. Soc, vol. xlvii, 1891, p. 266. 

 I I— Bull. Geol. Soc. Am., Vol. 3, 1891. 



