THE ROCKS OF PLYMOUTH. 215 
mineralogical and petrological questions by the preparation and 
examination of similar slides. 
The present paper contains a description of the leading rocks in 
the neighbourhood of Plymouth, based not merely upon their 
general external characters, but upon an examination of a large 
number of microscopic sections, specially selected and prepared. 
This field of local scientific research has hitherto been practically 
untrodden, and I hope therefore that the results of the enquiry may 
be of some interest and value. 
The title, “Rocks of Plymouth,” must be interpreted somewhat 
liberally. My aim has been to deal with the more characteristic 
rocks of the district between Brent Tor and the Eddystone in one 
direction, and from Ivybridge to St. Germans in the other. The 
land area included within these limits would be some 350 square 
miles. It is not to be understood that the whole of this district 
has been minutely inspected, but that as far as possible all the 
leading varieties of rocks therein have been collected and examined. 
Within a narrower area, however, a much closer survey has been 
made. Every point of special interest between the Notter and 
the Erme, and within these limits from Dartmoor to the sea, has 
been visited ; while for the more immediate vicinity of Plymouth 
itself the examination has been both close and protracted. 
The different geological periods reviewed range between the 
Archean, found in the Channel basin, and the Trias. Most of 
our area is occupied by Devonian strata, but on its northern 
boundary, near Brent Tor, there are Carboniferous. The granite 
is post-Carboniferous, and the Trias occurs at Cawsand, with a 
trace at Drake’s Island. These are the stratigraphical divisions as 
commonly recognised. I have a very strong belief myself, how- 
ever, that Silurian rocks are present in the metalliferous band of 
the Tavistock mining district, and am looking forward to a new 
Geological Survey on the 6+inch scale to solve this and sundry 
other local problems. In the very general absence of fossils over 
the greater part of the debateable area, petrology, by the careful 
analysis of lithological peculiarities, may render important aid. 
The divisions adopted in this paper are not, however, strati- 
graphical. Mr. Prideaux used a mixed topographical and group 
arrangement, and classed his illustrations under six heads—granites 
of Dartmoor, rocks bordering the granite of Dartmoor, rocks 
Q 2 
