142 
THE CAMBRIAN VOLCANOES 
BOOK III 
Hebrides and likewise part of the present western seaboard of Sutherland 
and Eoss. Along the margin of that northern land the white sand was laid 
down which now gleams in sheets of snow-like quartzite on most of the higher 
mountains from Cape Wrath to Skye. The sea lay to the east and, so far 
as we know, may have stretched across the rest of Scotland, and the north 
and centre of England. Another vestige of the land of this ancient era occurs 
in Anglesey. There, and likewise over scattered tracts in the Midlands, 
and in the south-west of England, the geologist seems to descry the last 
relics of islets that rose out of the Cambrian sea, and are now surrounded 
with its hardened sediments. 
Wliile such was the general aspect of the region of the British Isles 
during' Cambrian time, volcanic action manifested itself at various localities 
over the area, breaking out on the sea-bottom, and pouring fortli sheets of 
la\'a and showers of ashes, which mingled with the sand and silt that were 
settling there at tlie time. In the northern or Scottish tract no trace of 
this subterranean activity has been found ; but in the English Midlands 
and over much of Wales abundant evidence has been obtained to show that 
in those districts the Cambrian period was marked by frequent and pro- 
longed eruptions. 
As its name denotes, the Cambrian system is typically developed in 
Wales. It was there that Sedgwick first worked out the stratigraphical 
relations of its ancient sediments, and that Murchison demonstrated the 
succession of organic remains contained in them, applying to them the 
principles of classification laid down by William Smith in regard to the 
Secondary formations. It was there too that some of the earliest and 
most memorable achievements were made in the investigation of ancient 
volcanic rocks. Sedgwick and Murchison, besides the admirable work 
wliich they accomjilislied in establishing the stratigraphy of the older 
Faheozoic formations, clearly recognized that among these formations there 
were preserved the records of contemporaneous submarine eruptions. Sedg- 
wick showed tliat the mountainous masses of eriqitive rock in Horth Wales 
were really lavas and ashes, which had been discharged over the sea-floor at 
the time when the ancient sediments of that region were deposited, while 
Murchison established the same fact by numerous observations in the east 
and south of Wales, and in the bordering English counties. De la Beche 
had found similar evidence among the “ grauwacke ” rocks of Devonshire.^ 
Following in the track thus opened up by these great masters, the officers 
of the Geological Survey were enabled to unravel, as had never before been 
attempted, the complicated structure of the old volcanic regions of Wales. 
^ For early researches on the older Palaeozoic voh:auic rock.s of Britain, see Sedgwick, Proc. 
Gcal. Sot. vols. ii. (18-38) ]i]i. 678, 679, iii. (1841) p. 548, iv. (1843) p. 21.5 ; Q-mrt. Jowrn. Geol. 
Sot. vols. i. (1845), pp. 8-17, iii. (1847) p. 134. Murchison, Proc. Gaol. Sue. vol. ii. (18-33-34) ]i. 
85 ; Sihman System (1839) pp. 225, 258, 268, 287, 317, 324, 401 ; Siluria, 4th edit. 
(1867) p. 76 et seq. Do la Beche, Meih. Ocol. Survey, vol. i. (1846) pp. 29-36. A. C. Ramsay 
in the Map.s and Horizontal Soctions of Wales published by the Geological .Survey ; also 
Descriptive Catalogue of the Rook -Specimens in the Museum of Practical Geology, 1st edit. 
(1858), 2nd edit. (1859), 3rd edit. (1862); “The Geology of Korth AV'ales,” forming vol. iii. of 
Memoirs of t)ie Geoloyical Survey, 1st edit. (1866), 2ud edit. (1881). 
