THE ROCKS OF PLYMOUTH. ok 
among the slates near Lydford Gorge, that has become in parts 
almost wholly converted into this mineral. 
5. Felsite—Two sections cut from the Triassic trap of Cawsand 
do not afford much information. One was taken from a more 
earthy, and the other from a more vitreous variety, and the 
latter has a decidedly fluidal aspect. The base is felspathic, the 
colouring matter iron, and as a rule it has a dirty flocculent look 
under the microscope. There is a good deal more mica brought to 
light by the lens than is apparent to the naked eye, some 
decomposed, and some strongly dichroic. In addition to the mica, 
felspar is porphyritically developed, and in some cases has changed 
into felsitic matter; there is a little ilmenite. A group of felspar 
crystals, altered, in the centre of. the earthy section, has enclosures 
much muddled up with ferrite, and among them is what is 
apparently a needle of apatite. The rock may be called a felsite, 
but it appears to have been a basalt. The columnar structure it 
develops on Cawsand beach is one of the most notable features of 
our local petrology. 
GRANITES AND ELVANS. 
Granite.—It is not easy to improve on the general description 
of the Dartmoor granite, given by Sir Henry de la Beche: “As a 
whole, a coarse-grained mixture of quartz, felspar, and mica; the 
latter sometimes white, at others black, the two micas occasionally 
occurring in the same mass. It is very frequently porphyritic 
from the presence of large crystals of felspar, and here and there 
schorlaceous ; but the latter character is chiefly confined to the 
outskirts, where it adjoins the slates.”!_ The colour is commonly a 
bright-grey—or creamy-white—occasionally with a bluish tinge ; 
but it usually weathers to a ferruginous-brown, the tint penetrating 
some little distance from the surface, to an extent varying with the 
compactness of the texture of the rock. 
The most striking physical feature of granite is the manner in 
which it is jointed. All who are acquainted with the Dartmoor 
tors are familiar with the stratified appearance many of them 
present ; the mass is at times separated into tabular portions of 
considerable size, often into blocks which have the appearance 
of masonry, while occasionally a columnar arrangement is shown, 
1 Rep. Corn. Dev. W. Som. p. 157. 
R 2 
