ON THE CRYSTALLINE ROCKS 01' THK LI/AKl) DISTRICT. 470 



banded altered tuffs of similar compositiou. As regards the former, 

 some of tlie " eyed " hornblende-schists mentioned by one of us, as for 

 instance on the north side of l*orthoustock Cove, may be a porphyritic 

 dolerite which has been modified by pressure *, and converted into 

 a slightly foliated epidiorite. Other masses again, as in the upper 

 cliffs at Porthalla, are not at all banded and are even practically 

 without foliation. It is, however, difficult to attribute the mineral 

 banding and other structures in most parts of the mass to the 

 crushing or shearing of a holocrystalline rock. Is it then to be ex- 

 plained as a kind of fluxion-structure, as we have already done in 

 the case of the Granulitic Group? 8ome of the hornblende-schists 

 present a very close structural resemblance to certain hornblendic 

 bands in the latter group, and to some other rocks, hereafter to be 

 mentioned, which are undoubtedly igneous. Moreover, the mineral 

 banding — stripes consisting mainly of felspar or epidote alternating 

 with those mainly of hornblende — as at Cadgwith or to the S. of 

 Church Cove, would lend itself very well to this explanation. 

 Indeed, where the bands attain a considerable thickness, it is not 

 very easy to explain them by segregation during metamorphism f. 

 One case, indeed, where the thickness of the bands is perhaps at a 

 maximum for the district, seems to require the former exjjlanation. 

 A pit has recently been opened by the side of the road leading down 

 to MuUion Cove. The rock excavated is partly a coarse saussurite- 

 hornblende rock, without definite structure, partly a well-banded 

 variety of the same, some of the bands being full '3" thick : the one 

 clearly passes irregularly into the other. The former under the 

 microscope presents considerable resemblance to one of the east-coast 

 gabbros, for it affords the remains of plagioclase felspar, indications, 

 and in one case at least a remnant, of diallage, and even a sugges- 

 tion of the former presence of olivine. The banded variety contains 

 the same minerals, and bears considerable resemblance to the Jlaser- 

 gabbros described hereafter ; its structure does not suggest crushing, 

 and one or two of the thinner bands, where the two minerals are 

 smaller in size, present a very close resemblance to an ophitic 

 structure, in which there is a slight orientation in the felspars, 

 ^\e do not think it possible to explain this structure by the shearing 

 of a coarse holocrystalline mass. At any rate the rock must 

 originally have been a variety of gabbro J. 



But in some members of the Hornblendic Group we have to explain, 

 not only a banding, but also repeated resemblances to slightlv 

 irregular deposition, or even to " false bedding." 



We did our best, during our study of these rocks, to apply either 



* The larger felspars, which are rendered distinct by saussuritization, axe 

 occasionally partly idionioi'phic and seemingly crushed out. 



t I collected a specimen, in 1888, from the hornblende-schists of Sark (which 

 are practically identical with those of the Lizai'd), in which the bands attain a 

 thickness of -5".— T. G. B. 



I It might be urged that this rock did not belong to the hornblendic schists 

 as here defined. It agrees, however, macroscopically with them, and differs 

 rather markedly from the altered gabbros of the east coast, of which, moreover, 

 we have not seen any instances on this side of the district. 



