96 MR. B. E. WALKER ON THE GARNET-BEARING [Feb. I904, 



This mixture of a rhyolitic with an andesitic matrix explains the 

 peculiar chemical composition of these 'streaky' rocks. They have 

 silica-percentages varying from 63 to 69, which are much lower 

 than those of true rhyolites. The percentages of lime and mag- 

 nesia are also low, but are intermediate between the percentages of 

 these constituents in andesites and rhyolites. Potash preponderates 

 over soda, although the difference is not by any means so great as 

 in the typical rhyolites. 



The two following partial analyses were made : — 



A. B. 



Per cent. Per cent. 



SiO 68-89 6692 



TiO", ■ 



A1 2 3 \ 19-69 20-50 



Fe 2 3 J 



CaO 119 169 



MgO 020 0*22 



K,0 2-61 356 



Na 2 242 2-77 



Specific gravity, 2*679 at 18° C. 2704 at 12-7° C. 



A = Lowest ' streaky ' rock, west of Galleny Force. 



B = ' Streaky ' rock, 1750-foot contour, Whelter Crags. 



Ward gives the analyses of two rocks, one from Base Brown * 

 and the other from Slight Side ; the former has a silica-percentage 

 of 69-673, the latter 68-421. 



Mr. Harker" tabulates the silica-percentage of an Illgill-Head 

 ' streaky' rock as 69*48, and its specific gravity as 2*682. 



The specific gravities of ' streaky' rocks from Scawfell (3876) and 

 Base Brown were found to be 2*706 and 2*694 respectively. An 

 excellent example of a rock in which the andesitic and rhyolitic types 

 occur together, is afforded by a thin band of ' streaky ' rock found 

 below the main mass of ' streaky ' on Gait Crags. In it are lenticles 

 of colourless rhyolitic substance and brownish andesitic ash, the 

 latter containing many broken fragments of a labradorite- felspar. 

 A less convincing example occurs at the base of the High-Goat-Gill 

 series in the Haweswater District. 



So far, I have described ashes in which the 'streaky' character is 

 due to a large extent to lamination, but may also be produced by 

 pressure and by infiltration of various substances. That there are 

 also minor lava-flows in so great a thickness of rock seems pro- 

 bable. Ward pointed out the difficulty that would be experienced 

 in tracing such thin lavas. He did, however, map the compact 

 columnar rock on Base Brown as a lava. I have no slide of this 

 rock, and have never come across a rock in situ that I could call 

 a lava. 



A loose block found by Mr. Marr on the path to Sty-Head Tarn 

 from Seathwaite shows a lava-like character. It is a typical 

 ' streaky' rock, with dark-brown streaks and enclosed fragments of 



1 Quart. Journ. Geol. Soc. vol. xxxi (1875; p. 411. 



2 ' Chemical Notes on Lake-District Kocks ' in the ' Naturalist ' lor 1899, p. 56. 



