CHAP. XV 
THE DEVONIAN SYSTEM IN ENGLAND 
259 
sea can be shown to have spread was the south of England. To the north 
of that belt, the site of Britain during Devonian time appears to have been 
partly land and partly wide water-basins in which the Old Bed Sandstone 
was deposited. 
In that half terrestrial half lacustrine territory that stretched north- 
wards to beyond the Shetland Isles, many volcanoes were active, of which 
the chronicles will he described in later pages. The most southerly of 
these centres of eruittion j’et known was the district of the Cheviot Hills. 
Throughout the rest of England and Wales no trace of any contemporary 
volcanic action has been detected in the Old lied Sandstone. It is true 
that over most of that region rocks of this age have been concealed under 
younger formations. Yet throughout Wales, where the Old Bed Sandstone 
attains so vast a thickness, and covers so wide an area, it has not yet 
yielded a vestige of any contemporaneous volcanic eruptions. 
But over the sea-floor that covered the south of England, and stretched 
thence into tlie heart of Europe, abundant volcanoes have left behind them 
proofs of their activity. The first geologist who recognized these proofs 
and ti’aced their extent on the ground appears to have been De la Beche, 
who, by his detailed maps and careful description of tlie igneous rocks of 
Devonshire, did so much to advance the study of ancient vtflcanic action. 
This great pioneer not only determined the former existence of Devonian 
volcanoes, but he was likewise the first to detect and map tlie volcanic rocks 
associated with the Carboniferous and “ New Bed Sandstone formations of 
the same region. The broad outlines traced by him among the volcanic 
products of these three geological periods in the south-west of England 
still remain but little changed. Nor are they likely to be much improved 
until the ground is resurveyed on a larger and more accurate map, and 
with better petrographical etpupment than were available in his day. 
Not long after the observations of De la Beche came those of A. C. 
(rodwin-Austen, who devoted much time to a sedulous exploration of the 
rocks of South Devon, and satisfied himself that contemporaneous volcanic 
sheets were intercalateil among the limestones of that district. “ The coral 
limestones,” he says, “are in many places superincumbent on great sheets of 
volcanic materials, with which, in some instances, as at North Whilborough, 
they alternate.” He pointed out that the interstratified volcanic rocks are 
of two periods, one Devonian and the other Carboniferous.* 
In his Geological Maps of Devon and Cornwall, which are to the present 
time those issued by the Geological Survey, De la Beche made no attempt 
to discriminate between tlie varieties of igneous rocks, save tliat the liasic 
“ greenstones ” were distinguished from the acid bosses of granite and the 
elvans. But in his classic “ Beport ” much more detail was inserted, showing 
that he clearly recognized the existence both of volcanic ashes and of lavas, 
as well as of intrusive sheets. At the outset of his account of the “ Grau- 
wacke,” he remarks that the sedimentary deposits are accompanied with 
igneous products, “ a portion of which may also be termed sedimentary, 
* Trims. Geul. Soc. 2nd st'V. vol. vi. (1842), pp. 4(55, 470, 473. 
