ON THE CRYSTALLINE ROCKS OF THE LIZARD DISTRICT. 495 



The rock has a specific gravity of 2*59. The microscope proves 

 it to be composed of two imperfectly-mixed glassy magmas, exhibiting 

 very typically the fluxion-structure of a rhyolite. In transmitted 

 light one of these magmas is quite colourless, the other has a buff- 

 coloured porcellanous appearance. Under crossed nicols the slice 

 breaks up into a cryptocrystallino matrix, showing minute irregular- 

 shaped flecks of doubly-refracting matter. Here and there, more 

 particularly in the colourless portions of the magma, the matrix 

 becomes microcrystalline, showing very minute doubly- refracting 

 dots, presumably quartz, on a dark ground. The slice is dappled 

 with chlorite and contains some magnetite, ferrite, a misshapen 

 porphyritic felspar, and polysynthetic granules of quartz. 



The second dyke is at Housel Cove. This, in petrological 

 character, is closely related to the last one ; it cuts right across the 

 bedding of the hornblende-schists, and is in contact with these schists 

 along its western margin. On its eastern side it is separated from 

 the schists by a few feet of breccia, made up of fragments of the 

 felsite and doubtless of " mechanical " origin, for it is parted from 

 the solid rock by a fault, the walls of which are clearly indicated by 

 well-marked slickensides. On the western margin of the dyke the 

 hornblende-schists are somewhat crushed and rotten, and have 

 acquired by weathering a superficial resemblance to the felsite 

 breccia. 



Examined under the microscope, this rock has quite the aspe^ct of 

 a rhyolite, and its structure so closely resembles the rock above 

 described that the details would be a mere repetition of those already 

 given. Granules of quartz and felspar may be made out in the base 

 here and there, but they rarely present anything like crystallographic 

 outlines. The slices are sprinkled with leucoxene and they are full 

 of dots and strings of magnetite partially converted into ferric oxide. 

 Fluxion-structure is pronounced. In some cases the dots of iron 

 have been removed by aqueous agencies, giving the slice a pseudo- 

 vesicular character. The Housel-Cove rock contains more iron than 

 the Landewednack specimen, and so has a slightly higher specific 

 gravity, viz. 2*62. 



YII. Some Fragmentary Inclusions. 



(1) Fragment (about 3" in diameter) included in dioritic rock 

 (Granulitic Group), Kennack Cove. The rock resembles a horn- 

 blendic gabbro. It is ver}^ slightly streaky in structure. Under 

 the microscope it is found to consist of plagioclase felspar almost 

 replaced by the usual filmy decomposition-products, of aggregated 

 green hornblende, usually in rather small, rudelj^ shaped prisms, 

 some grains of brownish iron oxide, and a fair amount of sx)hene 

 and apatite. The gabbro-like rook on the south side of Porthoustock 

 Cove, it may be noted, also contains these two minerals, which, so 

 far as we have seen, are rare in, if not absent from, the ordinary 

 gabbro (that intrusive in the serpentine). 



(2) From a slab-like fragment, a few yards long and less than 



