510 MR. J. POSTLETHWAITE ON THE BIOEITIC PICEITE OF 



stituents are alike in both. There is a large boulder of the rock in 

 the bed of the stream, about 200 yards below the masses in situ, in 

 which some of the crystals are | inch in diameter. This variation 

 in crystalline structure is a feature which appears to be rather 

 common in rocks like picrite. The boulders scattered along the bed 

 of the stream, together with those used in the ' construction of 

 boundary walls, show that a large quantity of picrite has been re- 

 moved by the denuding agents that have scooped out the valley to 

 its present depth. 



Two slides, one prepared from the fine-grained picrite on Great 

 Cockup, and the other from the coarser rock on White Hause, were 

 amongst those examined by Prof. Bonney, and the following are his 

 notes on the slides and on the hand-specimens from which they 

 were taken : — 



" The specimens numbered 1 and 2 present a general resemblance 

 to those which I have from Little Knott, and I may refer to my 

 description already published ^ for the principal features, such as 

 the different varieties of hornblende, and their relations one to 

 another. In the new specimens it is more difficult to prove that 

 olivine has been a constituent of the rock, though I think it is 

 represented by some of the serpentinous aggregates; also there 

 seems to me a slightly larger proportion of felspar ; the rock, there- 

 fore, is rather farther away from a typical picrite than the Little 

 Knott rock, to which, however, it is closely related, and so it may be 

 regarded as one of the transitional forms between normal picrite 

 and normal diorite." 



About 300 yards east of the dioritic picrite, Hause Gill bends to the 

 north-east, and at this point, on the eastern side of the stream, there 

 is an exposure of rock, the true nature of which can hardly be deter- 

 mined without the aid of the microscope. In hand-specimens it 

 exhibits many of the characteristic features of a quartz-felsite, but 

 in other respects it is not very dissimilar from some of the grit 

 which occurs in that locality. The outer surface is much decom- 

 posed. Prof. Bonney has made the following notes on a slide pre- 

 pared from this rock : — 



" This rock has undergone so much secondary change that it is 

 difficult to speak positively as to the exact nature of the original 

 constituents. It is, however, undoubtedly igneous. The chief 

 constituent has been a felspar, which is now almost wholly replaced 

 by secondary products, the more abundant being rather fibrous 

 microliths, which give, with crossed nicols, fairly bright colours ; 

 with them are occasional spots and small patches of a clear 

 mineral, which gives low polarization-tints. It may be a secon- 

 dary quartz, or possibly a mineral allied to felspar ; occasionally 

 it is a crystalline grain or a small cluster of grains, resembling 

 dolomite. Moreover there are some patches, in fair quantity, 

 of a browner colour and ' dusty ' aspect, which are imperfectly 

 translucent, but also indicate the presence of secondary microliths, 



1 Quart. Journ. Geol. Soc. vol. xli. (1885) p. 511. 



