332 ME. ALFRED HARKER ON THE GABBRO [Aug. 1894, 



showing from 514 to 53-3 per cent, of silica. Prof. Bonney 1 

 pointed out the occurrence in these rocks of an altered rhombic 

 pyroxene. Mr. Teall 2 has given an account of one of the most 

 remarkable flows. The characteristic features are the occurrence 

 of large porphyritic crystals of a lime-soda felspar, frequently 

 rounded and having peculiar inclusions, and the abundance in the 

 groundmass of magnetite and pseudomorphs after hypersthene. 

 The metamorphosed lavas enclosed in the Carrock Fell gabbro have 

 a fresh aspect and a high density, the specific gravities of the 

 conspicuously porphyritic and the more compact types being 2*835 

 and 2-887, as compared with 2*754 and 2-744 at Eycott Hill. The 

 groundmass has become darker and more lustrous, and the large 

 felspars have a clearer appearance, though not otherwise altered to 

 the eye. Under the microscope it is seen that these felspars have 

 for the most part lost their conspicuous inclusions in the form of 

 negative crystals, but the crystals usually retain their identity, and 

 show their albite- and Carlsbad-twinning unaltered. Only occa- 

 sionally have they been recrystallized into a new mosaic [1550] in 

 the fashion that we have noticed in the basic lavas bordering the 

 Shap granite. The serpentinous or bastite-pseudomorphs after 

 hypersthene have been converted into a very pale, greenish 

 amphibole of rather fibrous structure. Possibly some of this 

 mineral may represent the original augite of the lava or its 

 decomposition-products, but the metamorphosed examples sometimes 

 contain fresh augite [1549, etc.]. The little twinned felspars of the 

 groundmass resemble those of the unaltered rocks, except in a greater 

 freshness and clearness, which could scarcely be interpreted as 

 conclusive evidence of recrystallization, but they sometimes appear 

 to fit together in the manner characteristic of metamorphosed rocks. 

 The magnetite seems to be on the whole in better octahedra than 

 in the unaltered lavas. But what points more unmistakably to 

 some degree of recrystallization in the groundmass is the disap- 

 pearance in the most altered rocks of the isotropic base. 



So far I have noticed familiar, and not even extreme effects of 

 thermal metamorphism in basic lavas. 3 There are, however, at the 

 actual junction of the lava with the gabbro, phenomena more 

 unusual, involving reciprocal modifications in the two rocks. I 

 have said that the line of junction can be shown in a thin slice 

 under the microscope, but it is often a curiously irregular line ; and 

 the plexus of small felspar-prisms, which constitutes a large part of 

 the groundmass of the lava, has been, so to speak, 'teased out' 

 at the edge, so that scattered prisms lie a little beyond what 



1 Geol. Mag. 1885, pp. 76-SO. 



2 ' British Petrography ' 1888, pp. 225-227. For a notice of the same lavas 

 as seen at Melmerby, across the Eden valley, see Quart. Journ. Geol. Soc. 

 vol. xlvii. (1891) p. 517. 



3 Some of the lavas, on the southern edge of the gabbro, show a different 

 type of metamorphism, and especially the development of abundant red 

 garnets. These phenomena, which are found in very many parts of the Lake 

 Disti-ict, have, I believe, no direct connexion with the gabbro intrusion, and 

 they will not be further referred to in this place. 



