PENGELLT — OIT THE AGE OF THE DAHTMOOR GEAXITES. 15 
thing granite which occurs in the true Dartmoor country. Sir Henry 
De la Beche says, " The granite of Dartmoor is, as a whole, a coarse- 
grained mixture of quartz, felspar, and mica, the latter sometimes 
white, afc 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, wiiere the Dart- 
moor granite adjoins the slates. The schorl not unfrequently occurs 
in radiating nests of variable size and abundance. A complete pas- 
sage may generally be traced between the compound of schorl and 
quartz, usually termed schorl rock, and the ordinary granite. The 
mica usually disappears as the schorl begins to be abundant, and 
sometimes, though not very commonly beyond limited areas, the 
granite is a mixture of mica, schorl, felspar, and quartz, in nearly 
equal proportions. After the absence of mica the next mineral 
which commonly disappears is the felspar, leaving the compound 
a mixture of schorl and quartz, the former sometimes occurring ih 
radiating nests in the latter; but more commonly the two minerals 
form an aggregate in nearly equal proportions." * 
This, though a comprehensive, is by no means an exhaustive de- 
scription ; considerable dissimilarity exists in the size of the aggre- 
gated crystals in different specimens ; nodules, apparently segregative, 
sometimes occurring in the substance of the ordinary granite, might, 
from the fineness of their grain, be almost mistaken for sandstone ; 
indeed, I not long since heard them appealed to as proofs of tlie 
metamorphic origin of granite. " Here," said tlie appellant, " are un- 
altered remnants of the old sandstone rocks, which, with these 
exceptions, metamorphism has converted into granite." I do not 
quote this for the purpose of endorsing it, but simply to show the 
general dissimilarity of the nodules to granite proper. Excepting 
their darker colour, they reminded me much of the granite veins 
which pass through the older granite of Goatfell, in the Isle of 
Arran ; nevertheless they are not veins but nodules, and capable of 
being extracted, as such, from the granitic mass containing them. 
Good examples of such nodules may be seen, amongst other places, 
at Shaptor. about two miles from Bovey Tracey, where I succeeded 
in extracting two good specimens. They consist of very fine grains 
of quartz and schorl, in about equal quantities, or with the latter 
somewhat preponderating. 
The observer who enters a Dartmoor quarry soon discovers that 
granite is by no means weatherproof; the eff<ect of the weather is 
very discernible, fully a foot or more within the exposed surfaces ; a 
more or less dark or ferruginous-looking band, of about the width 
just mentioned, graduates into the unchanged rock, and suggests 
that small fragments might, through long exposure and rough usage, 
undergo a very considerable change of aspect. 
The boulders which occur so abundantly in the beds of the Dart- 
moor rivers and rivulets are found to be more or less changed in 
* "llcport," p. 157. 
