512 ME. J. POSTLETHWAITE ON THE DIORITIC PICEITE OF 



proposed subdivisions of the Skiddaw Slate Series ^ crops out to the 

 surface, aud near the picrite it shows distinct traces of contact-meta- 

 morphism. The grit is a coarsely granular rock composed chiefly of 

 rounded grains, varying in size from yg- to ^ inch in diameter ; 

 amongst them are a large number of quartz and felspar-grains, 

 derived apparently from the waste of a granite or granitoid rock. 

 Skiddaw Slate is also represented by grains of both the smooth black 

 and light-grey varieties. In this rock, at a distance of about 300 

 yards from the picrite, on the north-western side, there are traces 

 of secondary change. Two slides prepared from the grit at this 

 point are described by Prof. Bonney as follows : — " No. 4. A fairly 

 coarse grit, consisting of quartz (apparently from a granitoid rock), 

 some felspar, in certain of which is a curious ' dendritic ' structure, 

 probably due to secondary change, bits of volcanic rock of a fairly 

 acid type, of grit, and of slate or phyllite. Some signs, but not con- 

 spicuous, of contact-metamorphism." The above notes refer to a slide 

 taken from the upper surface of the grit, at its junction with the 

 overlying soft slate. No. 5 is taken from grit of the normal type. 

 " This is certainly a fragmental rock, the constituents not very dis- 

 similar to those of the last (No. 4), but there are rather more 

 numerous fragments of acid lavas, and there is no well-marked 

 phyllite ; perhaps fewer quartz-grains. One fragment exhibits a 

 well-defined micrographic structure.'' 



I also had a slide prepared from a rather gritty slate which occurs 

 in the same locality, on which Prof. Bonney has written the following 

 note : — " No. 6 is a rather gritty slate, containing a fair amount of 

 small grains of quartz and flakes of mica (both fragmentary) mixed 

 up with the usual earthy material, like numbers of other not very 

 smooth- cleaving slates ; possibly a little affected by contact-meta- 

 morphism." 



The last three slides all show traces of secondary change, due to 

 contact with the dioritic picrite on Great Cockup. Some of the 

 soft smooth Skiddaw Slate on the north-western side of this ex- 

 posure of dioritic picrite also shows signs of contact-metamorphism, 

 in the form of alternating bands of light and darker colour. 



The amount of alteration observable in the rocks surrounding the 

 dioritic picrite, as the result of contact with it while in a molten 

 state, and during the process of cooling, is considerably larger than 

 that observable at Little Knott, although the amount of picrite 

 exposed in the latter locality is probably quite as large as in the 

 united areas on opposite sides of Hause Gill. We may infer, there- 

 fore, that there is probably, at a moderate depth beneath the sur- 

 face, a much larger mass of dioritic picrite than that now exposed 

 on the sides of Hause Gill. There is good reason for supposing 

 that the picrites at Little Knott and on the sides of Hause Gill are 

 of the same age, and, as Prof. Bonney suggests, probably came from 

 the same ' pot,' notwithstanding the distance by which they are 

 separated (2 miles) ; the trifling difference there is between the 



^ Ward's ' Physical History of the English Lake District,' Geol. Mag. for 

 1879, p. 122. 



