THE ROCKS OF PLYMOUTH. 229 
schist ; secondly, the alteration of the schist by heat into “ ribbon- 
jasper.” 1 
3. Dolerite.—The most characteristic of our dolerites occur in 
a series of bands on the west of the Tamar, near Saltash, and 
extending in part across the river to the eastward. The Geological 
Survey map makes no distinction between them and the lavas of 
the same locality ; but, save in their igneous origin, the two series 
are quite dissimilar. Considerable interest attaches to these 
dolerites, which have been made the subject in part of investiga- 
tion by Mr. J. A. Phillips. Being largely worked for road 
metal, they are the source of the highly crystalline fragments of 
dark heavy rock, so frequently to be found on the highways in the 
vicinity of Plymouth. 
One of the most compact, and microscopically the best preserved, 
is a crystalline variety from Treluggan—a heavy rock, very dark- 
green in mass, with dark sparkling crystals. It contains, among 
other minerals, hornblende, chiefly brown, apatite, augite, viridite, 
chlorite, and magnetite, in a felspathic base. The groundmass of 
the Weard rock is felspathic, with largely developed plagioclase, 
and disseminated through it are brown mica, hornblende, viridite, 
magnetite, apatite, epidote ; there is a little augite. The Burraton 
Combe rock is compact and tough, but not so well charactered in 
the section examined. It contains much altered ilmenite, and 
numerous needles of apatite. The Notter rock has a good many 
points of resemblance, with granular augite, a little hornblende, and 
plagioclase-felspar ; and in fact the differences are not greater than 
we should expect to find in parts of one eruptive boss. 
A peculiarly interesting dolerite occurs at Ernesettle. Stray 
surface boulders may be seen near King’s Tamerton, and while it 
weathers very brown and rotten, in its compact condition it has a 
mottled aspect—grey and blackish-green. It was found in situ in 
digging a well at Ernesettle, in spheroidal masses, and can be 
traced on the beach below, where its intrusion has altered some 
slate into a very fine-grained, compact, olive-grey rock, approxi- 
mating to Lydian-stone.? 
1 Much the same alteration has been produced in the rocks between the 
St. Cleer boss, already noted, and the Caradon granite; and it is also seen 
near Okehampton. 
2 Occasionally we find these intrusive bands give contact slates quite a 
flinty consistence. 
VOL. IX, R 
