THE ROCKS OF PLYMOUTH. : yi 
bell under the hammer. Whatever the variation in composition, 
they evidently belong to one series. 
An exposure of the same class occurs at Lydford, and there is a 
fine boss at Houndall, in Cornwood. The characters of these are 
first noted in this paper. The Hennock and Bottor rocks, near - 
Bovey, also appear to belong to this group, and there are similar 
rocks elsewhere on the borders of the granite. The suggestion 
from the phenomena they present seems to be that they were 
originally deep-seated dolerites, brought up on the flank of the 
granite in its upthrust, and subsequently exposed by denudation. 
They exhibit considerable alteration, and there is a develop- 
ment of hornblende which appears to be connected with the 
contiguity of the granite. Very much the same phenomena are 
exhibited in association with the Land’s End granitic area. 
The more distinctly-charactered gabbros come from Cocks Tor 
and Houndall. lLydford, Brazen Tor, and White Tor show more 
alteration in the development of hornblende. We have also 
hornblende-schists associated. 
The pyroxenic mineral in the Cocks Tor rock, which most 
observers regard as diallage, is indeed by some considered augite, 
in which case the rock would rather be a diabase. However, there 
is little reason to question its right to rank among the gabbros. It 
is a much altered rock, containing green pyroxene, in some cases 
partly converted into hornblende ; and the felspars are as a rule 
much decomposed, while the ilmenite is also decayed. Some parts 
of this rock are much fresher than others. The best local examples 
of diallage are perhaps to be found in the Houndall boss, in 
some portions of which this mineral is remarkably well developed. 
The sections examined contain much ilmenite and iron oxides 
partially decomposed (hematite and limonite?), some plagioclase 
lath-felspars, and needles of apatite. 
White Tor yields characteristic examples of the development of 
hornblende. A section cut from the summit contains both pale 
brown and green hornblende, diallage, and beautiful skeleton 
crystals of ilmenite. No felspar crystals are visible. There are 
some pale blue dichroic grains, which present the appearance of 
1 So does the intrusive rock of St. Cleer Down, near Liskeard, which has 
usually been classed as hornblendic, and contains exceptional proportions of 
that mineral. Its identity as a member of this series is however un- 
questionable. 
