396 Prof. T. G. Bonn ey. On Specimens of Rock [Jan. 28, 



presented me with two " vitrified " specimens of the so-called quartz- 

 felsite of St. John's Yale, together with some fragments of the 

 ordinary rock. All had been obtained from the quarry near Threl- 

 keld. The vitrified specimens, as he informed me, " had been thrown 

 with some fuel into the fire of the engine which propels the machinery 

 by which the felsite is crushed for road-metal." They had been picked 

 out from amongst the engine-slag, and had been given to him by 

 Mr. R. Humphreys, the late foreman of the quarry. Mr. Postle- 

 thwaite subsequently, on enquiry, at my request, was told that the 

 temperature of the fire would probably be not less than that of a 

 large locomotive (the engine is 70-horse power), in the hottest part 

 of which pig-iron melts. So that these specimens may have been 

 raised to a temperature of about 2000 F.* 



The rock is intrusive in Skiddaw slate, fragments of which it 

 includes, and which it slightly alters near the j auction. It is so well 

 known to geologists that we may refer for most particulars to the 

 Survey map and the memoir, f in which are given a brief description 

 (p. 33) and a small figure of the microscopic structure (Plate I, fig. 5). 

 The rock is a somewhat porphyritic micro-granite; the matrix is 

 holocrystalline, consisting chiefly of small grains of felspar and 

 quartz. The former are usually rather too much decomposed to 

 admit of the species being determined ; as a rule, they appear to have 

 consolidated before the quartz, since they are rectilinear (nearly 

 square) in outline, the other mineral filling the interspaces. The 

 quartz contains several minute fluid cavities, with relatively small 

 bubbles, and an occasional microlith or belonite, so small that any 

 attempt to determine it would be a waste of time. A fair number of 

 mica flakes, which sometimes run to a larger size, are also present. 

 Of these, the smaller occasionally appear to be moulded to the felspars. 

 Evidently they once belonged to the biotite group, but are now re- 

 placed, as is very common, by a pale-greenish, slightly dichroic, 

 secondary mineral a hydrous mica, or possibly a chlorite. There 

 are sparse granules of iron oxide and of (?) impure sphene. 



In this ground-mass are scattered larger felspar crystals, also more 

 or less decomposed, in which sometimes the striping of plagioclase 

 can still be distinguished, but at others the crystal is occupied 

 entirely by a minute fibrous or filmy secondary product, affording 



* Dana, ' Characteristics of Volcanoes,' p. 144, considers that the temperature of 

 molten basalt, such as that of Kilauea, is from 2000 to 2500 F. For complete 

 fusion of such a rock as the St. John's Vale quartz-felsite a higher temperature 

 would be required than in the case of basalt, but, as the Hawaiian lava is very 

 liquid, and some basic rocks, when still melted, are below 2000, perhaps the latter 

 of these temperatures might be sufficient. 



t 'Mem. Geol. Survey,' Lake District (by J. C. Ward), p. 8. See also a paper 

 by the same author, ' Geol. Soc. Quart. Jl.,' vol. 32, pp. 12, 22, &c. 



