237 



NOTES ON NORTH OF ENGLAND ROCKS. 



II. 



ALFRED HARKER, M.A., F.G.S. 



I PROPOSE in the present paper to describe a few more igneous rocks 

 presenting characters of interest to a student of petrology. The 

 numbers given in brackets [ ] refer to sHdes in the collection of the 

 Woodwardian Museum, but the rocks are taken from well-known 

 localities, and the descriptions will be found generally applicable. 



(vii) Hypersthene-quartz-gahbro of Carrcck Feli, Cumberland. — 

 This rock is the ' hypersthenite' of CHfton Ward, which occupies 

 a considerable area to the south of Carrock Fell. Dr. Trechmann 

 has pointed out that the dominant pyroxenic element seems to be 

 diallage, and he appears to question the occurrence of hypersthene. 

 Examination shows, however, that in various specimens this mineral 

 is nearly or quite as abundant as the diallage, and almost justifies 

 Mr. Ward's naming of the rock. 



Hand-specimens show a rather coarsely crystalline aggregate of 

 dark pyroxenes and dull white felspars, with little vitreous grey 

 patches of quartz. A slice [438] reveals the constituents and 

 structure of the rock. Felspar is abundant in crystals giving 

 rectangular sections and fine twin-lamellation : sometimes there are 

 cross-striae due to pericline-twinning as well as those following the 

 albite-law. The mineral belongs to labradorite, or a variety between 

 labradorite and bytownite. The rhombic pyroxene (hypersthene) is 

 almost invariably converted into a pale-green fibrous serpentinous 

 product — the so-called bastite — still preserving the rectangular con- 

 tour of the original mineral. This substance is distinctly pleochroic, 

 giving the strongest absorption when the length of the crystal is 

 parallel to the shorter diagonal of the polarising prism. The 

 hypersthene has evidently crystallised in general before the felspar, 

 and so shows good crystal outlines ; the diallage is of later consoli- 

 dation, and forms irregular plates moulding round the other 

 constituents. This diallage has a light-brown tint in section, and 

 exhibits a typical 'diallagic' structure with minute rod-like inter- 

 positions grouped parallel to two or three definite directions. It gives 

 the usual brilliant polarisation-colours between crossed Nicols. 



Another mineral of later formation than the felspar is a brown 

 dichroic mica, which, however, is often discoloured and partly 

 decomposed. The earliest products of crystallisation in the rock are 



August 1890. 



