THE ROCKS OF PLYMOUTH. 241 
have shown that the peculiarities of the Channel geology are by 
no means confined to this point, but that the bed of the Channel 
immediately off our coast contains a series of rocks which are 
quite as distinct from those of the mainland as those of the 
Eddystone, and have no relations on shore of even approximate 
resemblance. These Channel rocks vary from those of the shore 
chiefly in their Archean character, and appear to indicate that the 
Channel occupies the site of one of the most ancient land areas 
on the globe. Of this the Eddystone Reef is believed to be a 
fragment, and Professor Bonney also regards the metamorphic 
series of the Bolt Head as another remnant. The age of the Bolt 
rocks has been one of the most notable problems of our local 
geology, and they have been recently regarded as Devonian in 
their origin, and metamorphosed by the presence of granitoid rocks 
hidden beneath the waters of the Channel. But as the meta- 
morphism here is regional and not contact, which cannot be 
traced far from the vicinity of a directly-acting cause, the 
probability seems to be in favour of Mr. Bonney’s hypothesis. 
Now the gneiss of the Eddystone Reef, while associated with 
rocks of allied composition which have a more granitic aspect, is, 
as I have said, thoroughly typical and well charactered. There 
are also a few specimens of gneiss in existence, which are believed 
to have come from the Shovel Reef in the Sound, and to have 
been taken therefrom when the Breakwater Fort was built. The 
evidence has been questioned; but that the rock is gneiss, and 
that it belongs to this locality and was submarine, there can 
be no doubt. 
These gneisses are much alike. They are well foliated and 
contain garnets; and in addition to the usual constituents of 
quartz, felspar, and mica, we also have titanite, and probably 
zircon. The existence of the last two minerals here is known 
only through the microscope. 
True gneiss is not found elsewhere in this vicinity, though 
there is rock of gneissic aspect in the Bolt series; and gneissic 
rock occurs in Devon bordering the granite, and in Corn- 
wall at Treluswell, and elsewhere. I have noted micro-gneissic 
structure in a contact rock from Ivybridge; and some of the 
mica-schists of Meavy become gneissic by the development of 
felspar. Some of these rocks in fact may fairly be termed 
*‘cornubianite ”—a compact, granular, scaly variety of gneiss, met 
