96 
THE PERMIAN VOLCANOES 
BOOK VII 
many more places than those where they were noted on the older maps, and 
likewise extend for some miles further to the north and west. 
It now appears that in the central and chief district the lavas can be 
followed westward from Spray Down near Kellerton to Greenslade near 
North Tawton, a distance of about twenty-one miles. Their most northerly 
outcrop is at Thorn above Loxbere in the Tiverton district, and their most 
southerly visible portion passes under the Cretaceous rocks of Pen Hill. 
The distance between these extreme points is likewise about twenty-one 
miles. The whole display of volcanic phenomena is comprised within an 
area of less than 400 square miles. 
One of the most obvious features in this volcanic tract is the way in 
which the erupted materials lie along the lines of hollow or valley in which 
the red rocks were deposited. This is most distinctly exhibited in the central 
district. Here a belt of breccias and sandstones, varying from one to three 
and a half miles in breadth, runs for about five and twenty miles westward 
in a depression of the Culm -measures. At intervals, the lavas which lie 
near the base of the red rocks crop out along the margin of the belt 
throughout most of its extent. But they do not spread out over the older 
rocks, and they have evidently been erupted from orifices situated along the 
line of the valley. It is another example of the relation between the trend 
of hollows and the outbreak of volcanic vents, which I have referred to as 
so strikingly displayed in the distribution of the Permian volcanic rocks of 
south-western Scotland. 
The volcanic materials of the Devonshire Permian district consist 
mainly of lavas, but include also red sandy and gravelly tuffs. The 
whole volcanic group is remarkably thin, never attaining even the liniited 
development of the Ayrshire series. No adecpiate petrographical investiga- 
tion of these rocks has yet been made. Externally, as seen in the quarries 
and lanes, the lavas present the closest resemblance to those of the Permian 
basins of Ayrshire and Nithsdale. They show considerable differences of 
texture even within the same mass, some portions being dull, fine-grained 
purplish-red rocks, with scattered pseudomorphs of haematite and a few 
porpliyritic felspars, other parts passing into an exceedingly coarse amyg- 
daloid or slaggy pumice. Dr. Hatch, after a microscopical examination of 
a small collection of specimens, found that while most are olivine-basalts, 
containing ferruginous pseudomorphs after olivine (Kaddon Court, Pocombe, 
and near Budlake), others are true andesites (Ide, Kellerton Park) and even 
mica-tracliytes (Copplestone, near Knowle Hill). 1 As already remarked, 
some of the older writers mention the existence of quartz-porphyries. 2 
1 Geol. Mag. 1892, p. 250. The rocks have been more recently described by Mr. B. Hobson, 
Quart. Jour. Geol. Soe. vol. xlviii. (1892), p. 502. The rock of Kellerton Park is called by Mr. 
Hobson “ mica-augite-audesite,” and he gives a chemical analysis of it by Mr. E. Haworth, op. cit. 
p. 507. Mr. Watts has lately found one of the orthoelase rocks to be rich in olivine. 
2 See He la Beebe, Report, pp. 203, 204. My colleague, Mr. Ussher, found close to the Thurle- 
stone outlier of conglomerate near Kingsbridge, Devonshire, a small boss of quartz-porphyry 
which rises through and alters the Devonian rocks. The actual junction of this mass with 
the conglomerate is not seen, nor have any fragments of the porphyry been noticed among the 
pebbles. 
