454 JOURNAL OF THE PLYMOUTH INSTITUTION. 
coloured in the Geological Survey Maps as altered Devonian, which 
has considerable local interest. The point cannot be regarded as 
settled, but to me there seems little doubt, though the conclusion 
is oue rather of inference than evidence, that they are of Lower 
Silurian age ; that they form part of the same series as the rocks of 
the Doclman and Gorran Haven in Cornwall, which are admittedly 
Lower Silurian ; and that we must associate with them the Eddy- 
stone reef, which lies on the line of strike between the two, and 
the chief rock of which is of a metamorphic character. Mr. 
Pricleaux called it gneiss. If I am right in regarding the Eddy- 
stone as the link between those apparently detached Silurian 
fragments, we must fix the southern limit of the Devonian rocks of 
Plymouth at or near that point. The northern will be found in 
the Carboniferous rocks of Tavistock. The granite of Dartmoor 
occurs somewhat nearer on the east, its closest point being an 
isolated patch between Newnham and Hemerdon. 
And here let me correct a misconception which has been handed 
down from the infancy of geological science. Granite is commonly 
spoken of as a primary rock. It is nothing of the kind. There 
are granites of different ages, as there are slates and limestones. 
The Dartmoor granites are of three periods. The oldest is of later 
date than the Carboniferous rocks, through which the whole have 
been elevated ; therefore much inferior in antiquity to our own 
rocks here. The fact that granite pebbles of all three varieties 
occur in the Triassic Conglomerates shows that they are of older 
date than these beds. They may be of Permian age ; but all 
that we can say definitely is that they date somewhere between 
the Carboniferous and the Triassic periods.* We may, as I have 
said, treat of the Devonian rocks of Plymouth under three divisions. 
The underlying slates and associated rocks ranging downwards 
from the granite ; the limestones ; and the schistose and arenaceous 
overlying beds : the whole forming an ascending scale in the order 
named. 
The strata around Dartmoor, in nearly all directions, dip away 
therefrom. The rocks in this locality have a prevalent southward 
dip, gradually increasing in extent from the granite, and more 
frequently varying to the westward of south than to the cast. To 
this there are a few exceptions. Professor Phillips recorded in 
his " Pal a30 zoic Fossils" the existence between Tamerton and 
* This was first shown by Mr. Pengelly, fh.s. 
