THE ROCKS OF PLYMOUTH. 249 
L. Diabase, Rock.—Dark-green, coarsely crystalline, with 
handsome mottled appearance on polished face, Exhibits ophitic 
structure under the microscope, and consists of plagioclase-felspar, 
augite, ilmenite, serpentine, chlorite, viridite. The three latter 
are the products of the alteration of a magnesio-iron silicate, 
probably both olivine and enstatite, a few grains of the original 
mineral remaining unchanged. 
M. Diabase, Yarnham.—A dark, hard, fine-grained, compact 
rock, containing felspar, much decomposed, probably plagioclase, 
grains of augite and iron oxide, viridite, and veins of epidote, one 
of which is shown, traversing the slide. 
N. Vesicular-Lava, Brent Tor.—A brownish-purple rock, with 
vesicles, partly open, partly filled with quartz and calcedony, 
The matrix is opaque, broken by a number of clear microliths. 
O. Gabbro, White Tor.—A heavy, crystalline, dark-grey rock, 
black when polished. Diallage is predominant among the crystalline 
portions of the rock; but there is also some hornblende, a few 
grains of tourmaline, and some beautiful skeleton crystals of 
ilmenite, one of which is figured. 
P. Prase-Schist (ribbon-jasper), Peek Hill—A compact quart- 
zose rock, with flesh-coloured and green laminations. Under 
the microscope the mass of the rock is seen to be dark-green, 
partially mottled ; on the edges this green matter forms delicate 
bladed crystals of actinolite. This is probably the result of the 
alteration by the granite of an adjacent hornblende-schist, as at 
Ivybridge, described under the next head. 
Q. Hornblende-Schist, Ivybridge.—Dark grey, fine-grained, with 
light veins or laminations. The microscope shows that it consists 
of quartz foliations, and of bands which contain numerous flecks 
of pale-brown hornblende, mostly minute. Between this rock 
and the granite, as at Peek Hill and elsewhere, is a mass of 
*¢ yibbon-jasper.” 
R. Altered-Slate, Shaugh.—This shows some of the early 
stages of alteration produced by the granite. Macroscopically it 
has a very silky sericitic texture. The microscope reveals the 
existence of groups of bladed crystals of andalusite, and of a 
large number of clear microliths. 
