226 JOURNAL OF THE PLYMOUTH INSTITUTION. 
INTRUSIVE IGNEOUS ROCKS. 
We pass next to the intrusive rocks. They are readily dis- 
tinguishable as a rule from the lavas by their darker hue—green 
predominating, and by their markedly crystalline texture. They 
are the distinctive ‘greenstones” of the older geologists. They 
are not of such frequent occurrence as the lavas; they do not 
graduate into the contiguous rocks, though at times they produce 
considerable alteration; their own boundaries as a rule are well 
defined ; they are not associated with ashes or tuffs; they contain 
a greater variety of minerals, and although in several instances 
much altered, their structure is commonly more distinct. They 
may be divided locally into five groups. 
1. Picrite-—The one distinctly peridotic rock of this locality, 
and the only one therefore which fairly represents in our neigh- 
bourhood the olivine group, with which Professor Bonney com- 
mences his new rock classification, is the boss of Clicker Tor. 
This is now commonly regarded as a serpentine, and was originally, 
according to Mr. Allport, an olivine-dolerite.! The olivine is 
nearly all converted into serpentine, and the process of conversion 
has taken place in the usual semi-reticulated fashion, which makes 
a section an exceedingly beautiful microscopic object. The mass of 
the rock is thus now serpentine, and the olivine chiefly represented 
by pseudomorphs, but the general structure is thoroughly charac- 
teristic. Augite and magnetite are also present. 
2. Gabbro.—To the north-east of Tavistock, at Cornwood, and 
elsewhere on the borders of the Moor, we have a set of exposures 
of a somewhat remarkable group of rocks. The Tavistock division, 
which are best known, occupy in part what may be called a bay 
in the granite, ranging north and south, from Brazen Tor to 
Cocks Tor.. They are described by Mr. Rutley in his ‘ Eruptive 
Rocks of Brent Tor and its Neighbourhood,” as consisting mainly 
of gabbro, with some amphibolite, and possibly some diabase. 
The best charactered are hard, tough rocks with a sub-metallic 
appearance, and some loose, scattered blocks ring almost like a 
1 Mr. J. H. Teall, F.G.s., who figures it in his magnificent work on 
“ British Petrography,” suggests the use of the name “Picrite.” 
