466 JOURNAL OF THE PLYMOUTH INSTITUTION. 
All these rocks have undergone great changes since they were 
first deposited. We have no sedimentary rocks in this locality, 
setting aside the hypothesis of Mr. Jukes, of Carboniferous date. 
If there were ever any they have been removed ; and although it 
is by no means improbable that the great culm trough of Central 
Devon thus far overlapped its borders no evidence remains ; unless 
it be derived from those diorites which seem to be intrusive, and 
may be of Carboniferous time. When, however, at the end of the 
Carboniferous period the granite of Dartmoor was upheaved, bear- 
ing on its flanks the rocks around, the effects of that upheaval 
must have cumulated here, especially if the Silurian belt to the 
south was upheaved in a contrary way — perhaps, as Dr. Holl sug- 
gests, by deeper seated granite — or, little affected itself, acted as a 
buttress against which the Devonian rocks were thrust ; while the 
superior rigidity of the limestone over its associates would increase 
the lateral pressure from the north on the rocks of my third division, 
lying between. This of itself would account for much of the con- 
tortion and faulting that prevails. 
Secondary Rocks, &c. 
Eut we do not stop here. There is the clearest evidence in the 
existence of outliers that the Triassic rocks once occupied a much 
larger area in Devon than they do now. There is one such outlier 
in Eigbury Eay, at Thurlstone. And here, at Cawsand, is a fels- 
pathic trap identical in character with the trappean rocks of the 
Trias, and manifestly intrusive, " breaking through the older red 
deposits." The rock is porphyritic, the base "a somewhat earthy 
compound of felspar and quarts, containing crystals of mica and 
(more rarely) felspar."* The intrusion of this rock must have 
been accompanied by an enormous amount of distortion and dislo- 
cation, and in its turn have aided the phenomena at Staddon. 
It is evident that from time to time this locality came within 
the influence of volcanic centres of considerable activity. JNor is 
it at all improbable that the outlines of the Sound were originally 
sketched by these mighty forces, qualified in their action by the 
differing resistile powers of the rocks. The limestone was one of 
the most rigid; but its resistance after all was limited. If not 
bent, it would be broken ; and fissures were opened therein that, 
De la Beche " On the Formation of Rocks in the South-West and South 
of England," p. 259. 
