240 JOURNAL OF THE PLYMOUTH INSTITUTION. 
approach to it in a rock of the Plym Valley, which contains crystals 
of gilbertite, the micaceous constituent of the St. Stephens rock. 
St. Budeaux.—There is a boss of felspathic rock near St. 
Budeaux wholly unlike any other in the district, which falls into 
place best here. It occupies a very small area on the crest of the 
hill immediately west of Kings Tamerton, on the road leading from 
Saltash Bridge to St. Budeaux. It is a massive felspathic rock, 
with well-developed twinned orthoclase crystals, traversed by 
numerous needles of schorl. These are long slender prisms, and 
much decayed, so that under the microscope they appear to be 
largely resolved into a dark-green decomposition product. The 
existence of this rock was noted by Mr. Prideaux ; and so far as 
I am aware, it stands alone in the immediate locality. It seems 
to indicate the near neighbourhood of true granitoid, if not 
absolutely granitic masses. 
METAMORPHIC ROCKS. 
All rocks are more or less changed, but the term metamorphic 
is by common consent applied to those in which the original 
characters have ceased to be prominent, or disappeared altogether. 
Two great classes of altered rocks are recognized—those which 
have been changed by causes affecting large areas—regional- 
metamorphism ; and those which have been changed by a directly 
acting cause—contact-metamorphism. Representatives of both 
classes occur in the Plymouth district—the one in the gneisses 
and allied rocks of the Channel area; and the other specially 
in the belt of rocks girdling the granite. It is convenient to treat 
these altered rocks as a distinct class, but the practice is hardly 
scientifically defensible. 
Gneiss.—We commence our review of this section with the 
Eddystone Reef, which is mainly formed of the foliated rock 
called gneiss. 
This gneiss has been familiar to geologists for many years. It 
is thoroughly typical in its character, and the most recent research 
has shown that it is one of the very oldest rocks in the kingdom, 
belonging to the most ancient of all recognized formations—the 
Archean. Nor is this all. A very remarkable series of investi- 
gations, carried on by Mr, A. Roope Hunt, r.us., r.a.s., of Torquay, 
