538 W. M. Eutchings — On some Lake- District Rocks. 



brought away is 2 74, and the silica-percentage is 60-45. Eoth 

 gives as the outside figures for silica recorded for dacites, 68-18 

 per cent., and 55-91 per cent, respectively, the latter being from an 

 altered rock. It is not likely that the figure obtained by me for 

 this Lake District dacite quite represents the original percentage, 

 which, judging by the alterations, was probably a little higher, 

 though certainly not nearly high enough to place the rock among 

 the rhyolitic andesites. "We have here, then, a well-characterized 

 biotite-dacite, and it is likely that this or a similar rock will be 

 found exposed at other points besides the one indicated. 



Another rock occurring above Easedale Tarn, Grasmere, on the 

 side towards Langdale, is a conspicuous representative of the other 

 extreme of the series of Lake District rocks. It is a dolerite, a good 

 deal more basic than any described by Mr. Ward, and differing in 

 other ways from any hitherto recorded in the district. It is a very 

 dark-coloured rock, easily recognized with a pocket-lens as a rather 

 coarse-grained dolerite, without any porphyritic structure. Micro- 

 scopic examination shows it to be made up of augite and plagioclase- 

 felspar, with usual altei-ation-products. Augite is abundant, and 

 much of it is still quite fresh. It existed originally, apparently, 

 wholly as irregular grains, mostly of good size. The rock has 

 undergone considerable crushing and shearing, and many augite- 

 grains are cracked in several places, the cracks being cemented with 

 chlorite. In some cases large grains have been completely shattered 

 into small fragments, now lying among chlorite and calcite, due 

 to subsequent infiltration. Here and there augite is fringed with 

 secondary hornblende, but chlorite is the more usual alteration- 

 product. 



The felspar is in the form of relatively broad laths, tabular 

 sections, and many irregular grains; it has been a good deal affected 

 mechanically, like the augite, but is very fresh otherwise. There is 

 no ophitic structure and no trace of ground-mass of any description. 

 The rock is distinctly a holocrystalline, non-porphyritic dolerite, 

 verging towards granitoid structure. The sp. g. is 2-95, and the 

 percentage of silica is 45-65. 



Another totally different type of basic rock examined is from near 

 the summit of Scarf Gap Pass. Mr. Ward mentions a bed of basic 

 lava on the east side of the pass, and gives description and figure of 

 it. I do not know whether this is the same rock, and his figure is 

 of very little use in deciding, while some points in his description 

 do not agree with the occurrence here noticed. 



It is a vesicular rock, the vesicles being now infilled with 

 chlorite and other alteration-products. It has comparatively few 

 porphyritic felspars, all very much altered. There is abundant 

 porphyritic augite in good-sized crjj^stals and grains, much of which 

 is still perfectly fresh, though much fractured in some cases. The 

 ground-mass consists of very small felspar-laths, perfectly fresh, 

 nearly all binary twins. They are mainly if not wholly labradorite. 

 There is the usual abundance of alteration-products throughout the 

 ground-mass, but a good deal of augite in small grains is still dis- 



