THE ROCKS OF PLYMOUTH. 239 
Greisen and Pegmatite.—Greisen, which is a mixture of quartz 
and mica, occurs at Saddleback and near Cornwood, and probably 
is not infrequent along the border line; and pegmatite—quartz 
and felspar, in large crystals—is found below Trowlesworthy, and. 
carried by the river as far as Plym Bridge, some examples with red 
felspar being very characteristic. Occasionally this rock is seen 
in veins traversing the slate. An even-textured granular peg- 
matite occurs at Chagford, and a very handsome form in the bed 
of the Erme, below Ivybridge, with a schorlaceous variation near 
the granite. Fine-grained pegmatite (aplite) is found at Brazen 
Tor, and some has been washed for many miles down the Tavy. 
Trowlesworthite—The most remarkable rock of this locality is 
that which I discovered on Trowlesworthy Tor, and named from 
its habitat Trowlesworthite. It is mainly a compound of felspar, 
tourmaline, and fluor, with a little quartz in occasional grains. 
The habits of the tourmaline in this rock and in Luxulyanite 
are exactly similar. Professor Bonney, who kindly examined it, 
has come to the conclusion that it is an altered rock formed 
from the normal granite of the district—the mica having been 
replaced by brownish tourmaline, some portions of the felspar by 
greenish-black tourmaline and quartz, and the original quartz by 
fluor-spar. There is a fluoriferous granite with which some 
petrologists have hastily confounded this Trowlesworthite ; but 
up to the present moment it remains true that this rock has only 
been found in this one place, in only one block; and that all the 
specimens known have therefore passed through my hands. 
Kaolin-Rock.—As a form of granitoid rock, and one of the 
most commercially important, we should note here the china-clay 
deposits of Lee Moor, and other localities, in which the felspar of 
the original rock has been changed into kaolin by a process very 
imperfectly understood. This must of course be regarded as an 
altered rock ; but it is not altered in the same sense as the meta- 
morphosed rocks to which we shall shortly have to allude. In 
some instances this change takes a somewhat different turn, and 
for kaolin we have the indurated variety, lithomarge, which I 
have found at Peek Hill. The quartz is also occasionally replaced 
by opal. 
China-stone is not found in our district, but there is a decided 
