220 JOURNAL OF THE PLYMOUTH INSTITUTION. 
them with that singular peak of Brent Tor, which is really the 
degraded stump of an old-world volcano, and consequently itself 
partly intrusive, partly contemporary. 
2. The intrusive boss of Clicker Tor, originally an olivine- 
dolerite—that is, a dolerite which, in addition to its normal 
constituents, contained olivine—now by alteration a serpentine. 
This is not the only rock of the district, however, in which the 
change from olivine to serpentine occurs, though it is the only one 
to which the name picrite, suggested by Mr. J. H. Teall, can be 
applied. 
3. A series of intrusive rocks near the borders of the granite, 
chiefly identified hitherto in the valley of the Tavy, but ranging 
over a much wider area. The leading constituents are plagioclase- 
felspar and diallage, and they are therefore classed as gabbros. 
Hornblendic forms also occur. 
4. A series of deep-seated intrusive rocks, which may be 
regarded as the “‘greenstones” proper, so far as that term has any 
definite meaning, and which for the most part are ancient 
dolerites ; that is, they are rocks in which the leading constituents 
are or were plagioclase-felspar and augite. These are now in 
part hornblendic. 
5. Some exposures of intrusive rocks which display much 
alteration. Once dolerites or basalts, they are now, by the 
development of chlorite, diabases. 
6. An intrusive rock on the shore of Cawsand Bay, associated 
with the remains of the Triassic outlier, undoubtedly of Triassic 
age, and in all probability the youngest of our igneous series. 
We deal with each group in order. 
CONTEMPORARY IGNEOUS ROCKS. 
Modern volcanic rocks are divided into three great orders—the 
vitreous, the crystalline, and the fragmentary. Representatives of 
all three are found among the ancient lavas and ashes of South 
Devon and their allies. Even the least informed note the 
resemblance which the highly-vesicular rocks of Brent Tor bear to 
modern pumice. The pumiceous character of the form of dun- 
stone commonly known as “honeycomb-dun” may not be so 
readily apparent, but occasionally it is nearly light enough to float 
upon. water. 
