ON THE CRYSTALLINE ROCKS OF THE LIZARD DISTRICT. 491 



not exceed 1 foot in thickness, and is bordered apparently by rotten 

 serpentine ; but within a tew inches on the south side is a thin 

 mottled band, which may be a rotten gabbro. The rock externally 

 is generally light-coloured, but within is seen to consist of a smoke- 

 grey coloured t'elsi)ar, with slightly oily lustre, probably labradorite, 

 in fairly large crystals, and of a few conspicuous flakes of a dark mica. 

 Its microscopic structure is difficult to describe. There are some 

 flakes of mica — one or two being biotite, the rest a white mica, with 

 inclusions of iron oxide between the cleavage-planes, and so probably 

 a " bleached " biotite ; one or two tufts of a nearly colourless mineral 

 in more or less acicular fibres, most likely actinolite ; a grain re- 

 sembling a serpentinized olivine (it is not quite normal in character), 

 the rest being a closely connected group of minerals, felspars or their 

 alteration-products. This consists partly of fair-sized grains of a 

 felspar which, in general appearance and extinction, corresponds 

 with labradorite, but does not exhibit the usual oscillatory twinning : 

 partly of a mosaic of crystals and crystalline grains, which in places 

 assume the " saussurite " condition already described. The mode of 

 occurrence and association of these with the larger crystals does not 

 suggest that the latter have been broken up by mechanical pressure ; 

 rather that parts of them have undergone a molecular re-arrange- 

 ment. They project irregularly, sometimes with rectilinear outlines, 

 into the larger grains, and sometimes a single grain or group of 

 grains, with either form of outline, appears insulated in the felspar, 

 like an island near a coast-line. One is reminded of the formation 

 of scapolite from labradorite described by Prof. Judd, though these 

 grains are not the former mineral, but seemingly are also felspar. 

 Cleavage-planes can be seen, and the line of extinction makes with 

 them angles varying up to at least 30°. 



Y. Manacle Point and Porthoustock Cove. 



Prof. Bonney was unable, when working for his former paper, to 

 make more than a hasty traverse of the rocks of Manacle Point, so 

 we examined the coast-sections from a cove on the south to 

 Porthoustock Cove *. Over the greater part a gabbro dominates, 

 generally about as coarse as the normal rock of Crousa Down, though 

 occasionally a very coarse variety is found. Sometimes also it 

 becomes rather fine-grained, the change from one to the other being 

 often fairly rapid. A foliated structure occurs, though but rarely. 

 As at Pen Voose, the gabbro is broken into by a granular rock, 

 sometimes porphyritic, sometimes green-spotted, sometimes dull 

 greyish and speckled. Both these are cut by a rather compact 

 greenstone, which is occasionally slightlj'" porphyritic ; of this not 

 much is seen on the south side of the Point, the quantity seeming 

 to increase as we go northwards t. 



The south side of the actual cove at Porthoustock is very puzzling, 

 and in places the crags could only be examined from a boat in very 



* That is, the whole area coloured as ' greenstone ' in the Surrey map. 

 t We reserve the details of these rocks for the next section. 



