THE DARTMOOR VOLCANO. 
161 
We find similar indications in the ancient detritus preserved in 
the Triassic conglomerate. While granites are few, elvans abound, 
and rhyolites (using that term in a wide general sense) are fairly 
prominent — precisely what we should expect to result from an 
early stage of denudation, when the rocks dealt with were 
generally of the middle and surface types, and the area of 
granite exposed was comparatively small. The Teignmouth 
conglomerate has yielded trachytic rock with large vesicles 
elongated in the direction of flow, more ordinary rhyolites, 
perlitic felsite, vesicular felsite, mica-andesite, and a wide range 
of elvans — granular, compact, and porphyritic — some identical in 
character with varieties occurring on the Moor in situ now. 
It is further noteworthy that the great bulk of the granitoid 
pebbles found on the beaches of the south coast of Devon are 
felsites, or elvans. Making every allowance for the frequently 
superior lasting powers of compact over granular rocks, we see 
here also that the felstones must once have been far more 
abundant in the moorland area than now. I have noted this 
at Slapton, near the mouth of the Ernie, and elsewhere ; and 
a number of granitoid pebbles, collected by my son at Grey 
Lake Cove, outside the mouth of the Yealm, proved to be 
all more or less felsitic. The modern river beds of the Moor 
present very different characteristics ; yet the ancient detrital de- 
posits in the bottoms are often as markedly elvanitic, telling 
the same tale as the beaches. But the most interesting facts 
under this head have yet to be noted. The fortunate discovery by 
Mr. E. Burnard of a very ancient detritus at Cattedown, clearly 
in its main features of Dartmoor origin (but so old that it carries 
us back to a time when Cretaceous and Liassic rocks still existed 
on the southern flanks of the moorland region, within the water- 
shed of the Tavy and the Plym), has yielded examples of volcanic 
rocks hitherto unknown in this county. There are two varieties 
of andesite, which Professor Bonney says closely resemble those 
of the Andes ; and a volcanic grit, which he regards as one of 
the most remarkable rocks he has ever seen, and which he has 
very kindly described as follows : 
"The rock is composed of more or less rounded fragments, 
cemented by a little 6 paste/ which is probably quartz, sometimes 
clear and chalcedonic, sometimes crowded with dust-like particles. 
Some of the fragments are felspar, fairly irregular in outline, in 
