THE ROCKS OF PLYMOUTH. 237 
A close-textured, vitreous, blue-grey, elvany rock at Lee Moor 
contains porphyritic quartz. 
An elvan at Grenofen is a very handsome rock, but unfortunately 
does not polish well. The base is of a fine-grained granitoid - 
character, largely composed of quartz and mica, and it contains 
porphyritic felspars, pink, brown, and white. ‘The presence of 
chlorite gives some portions a green cast, and it also in parts 
reveals pyrites. Mr. Rutley states that it contains hornblende, 
which would make it a syenitic-elvan. He regards it as a fine- 
grained porphyritic granite, with a little felsitic material; but 
adds that it may also be regarded as a quartz-porphyry. It serves 
very well to illustrate the difficulty there often is in classifying 
our local granitoid rocks apart from their behaviour in the field, 
on one hand, or their microscopic structure, on the other. As we 
have seen, the schorlaceous-elvans especially pass with ease into 
schorlaceous-granite. 
Peek Hill yields a granular elvan, with patches of schorl. 
A dyke near Modbury, classed by De la Beche as an elvan, is 
mainly quartzose, hard and crystalline. 
Schorlaceous Rock.—We pass to a class of rocks that, while 
generally granitoid in character, deviate more or less from the 
granitic or elvanic type. 
Schorlaceous-granite, already mentioned, is simply granite which, 
in addition to the ordinary constituents, contains varying quantities 
of tourmaline or schorl, occasionally amorphous, or granular, but 
usually crystallized, not unfrequently in long, radiating, hexagonal 
needles. This granite, as Sir Henry de la Beche says, is commonly 
met with on the borders of the Moor, and may often be traced 
passing into schorl-rock, by the disappearance first of the mica and 
then of the felspar. Examples are to be found, however, of distinct 
junctions between schorlaceous-granite and schorl-rock, in such a 
way as to indicate that the latter is the product of a distinct 
_ antecedent. The same point may be observed in the junction, 
clear and distinct, of ordinary schorl-rock and schorl-schist—which 
is made up of foliations of schorl and quartz—showing that the 
two have not one common origin, though their present condition 
may probably be due to the same agent and period of alteration, 
This fact does not seem to have come under the notice of De la 
Beche ; but it is one of considerable importance, and indicates the 
