CHAP. XV 
THE DEVONSHIRE ERUPTIONS 
261 
absence of igneous rocks in North Devon, a strip of country extending from 
Newton Abbot and Torquay westwards by Plymouth across Cornwall to 
Penzance contains abundant records of volcanic action. It lias not yet 
been possible to map out, among what were formerly all grouped together 
as “ greenstones,” the respective limits of the bedded lavas and the tuffs, to 
distinguish the true sills, and to fix on the position of the chief vents of 
eruption. So intense have been the compression and shearing of the rocks 
that solid sheets of diabase have been crushed into fissile schists, which can 
hardly be distinguished from tuffs. Moreover, owing perhaps in large 
measure to the mantle of red Permian (or Triassic) strata, which has been 
stripped off by denudation from large tracts of tliis region once overspread 
by it, the Devonian rocks have been deeply “ raddled, or stained red. But 
probably one of the main sources of difficulty in studying the petrography 
of the area is to be found in the results of atmospheric weathering. Devon- 
shire lies in that southern non-glaciated strip of England, where the rocks 
have been undergoing continuous decay since long before the Ice Age. 
No ploughshare of ice has there swept off the deep weathered crust, so as to 
leave lim’d surfaces of rook, fresh and bare, under a protecting sheet of 
boulder-clay. It is seldom that a really fresh piece of any igneous rock 
can be procured among the lanes and shallow pits of Devon, where alone, 
for the most part, the materials are exposed. 
Much, therefore, remains to be done, both in the stratigraphy and 
petrography of the Devonian volcanic rocks of this country. To the late 
J. A. Phillips geology is indebted for the first detailed chemical and 
microscopical investigation of these rocks. He clearly showed the truly 
volcanic origin of many of the so-called “ greenstones.” He believed that 
certain “ slaty blue elvans,” which he found to have a composition 
identical with that of altered dolerites, might be highly metamorphosed 
tuffs, and that others might have been originally sheets of volcanic mud. 
After studying the chemical composition and minute structure of a large 
collection of “ greenstones,” he demonstrated that in all essential particulars 
they were dolerites, though somewhat altered from their original character. 
Subsequently they were studied by Dr. Hatch, who found the fresher 
specimens generally to possess an ophitic structure, while some are granular, 
others porphyritic." 
Although the rocks have undergone so much crushing, solid cores ot 
them, showing the original structure, may be obtained, also examples of the 
amygdaloidal, vesicular or slaggy character. They occur in sheets either 
singly or in groups, and appear generally to be regularly iuterstratified in 
the slates and grits. While some of these intercalations, especially the 
amygdaloidal sheets, may be true superficial lavas, it can hardly be doubted 
that others are sills, especially those which assume the crystalline structure 
^ See especially Quart. Journ-. Geol. Soc. vols. xxxii. and xxxiv. ^ ^ 
2 A few of the eruptive rocks of Devonshire have recently been studied by K. Busz. He 
finds most of his specimens (chiefly from the Torquay district) to be varieties of diabase, but 
describes a palaeopicrite from Highweek near Newton Bushel, and a kersantite fiom outi 
Brent on the south-east edge of Dartmoor (jVewes Jahrh. 1896, p. 5/ ). 
