486 Geological Sociely. 
this formation, whilst the marly sandstones and conglomerates of 
Herefordshire are abnormal exceptions in it, we see the reason why 
their slaty continental equivalents, like the greater part of the South 
Devon slates, have been referred to the undivided Wernerian forma- 
tion of grauwacke. 
Mr. De la Beche in his map of Devon and Cornwall, published 
in 1839, has adopted divisions of the strata, similar to those of Pro- 
fessor Sedgwick and Mr. Murchison, as to their order of sequence ; 
applying, provisionally, to the culmiferous rocks the name of Car- 
bonaceous series, and to the Devonian and Cornish slates the appel- 
lation of Greywacke. 
We know also on the authority of Mr. De la Beche that tin mines 
are worked in carbonaceous rocks at Owlescomb near Ashburton, 
on the east side of the Dartmoor granite, and on its west side at 
Wheal Jewel near Tavistock. He further informs us that one of 
the richest tin mines now worked in Cornwall, namely the Charles- 
town mine, east of St. Austle, is in a fossiliferous rock containing 
Encrinites and corals, and that the same corals oecur also near tin 
mines at St. Just; and in the neighbourhood of Liskeard the Rev. 
D. Williams has found slates which contain vegetable impressions, 
dipping under other slates which are intersected by lodes of tin and 
copper. 
From these new facts, we learn that the killas and other slate 
rocks of Cornwall and the south of Devon do not possess the high 
antiquity which has till lately been imputed to them; and that tin 
occurs, as copper, lead and silver have long been known to do, not 
only in slate rocks that contain organic remains, but even in the 
coal formation. 
Mr. Greenough, in the new edition of his map of England, repre- 
sents nearly the same boundaries and order of succession in Devon 
and Cornwall as we find in the maps of Mr. Dela Beche and Messrs. 
Sedgwick and Murchison ; but in his memoir connected with the 
map, adopting the name of Carbonaceous series for the culmife- 
rous rocks, he substitutes that of Upper killas for the Devonian 
system of Sedgwick and Murchison, (including under that term 
the old red sandstone of Herefordshire,) and Lower hillas for the 
slates inferior to the Silurian system, which they have termed Cam- 
brian. 
Mr. Greenough, in his memoir, also shows by quotations from Dr. 
MacCulloch, that the undisputed old red sandstone of the north of 
Scotland exhibits, at intervals, the same great changes of mineral 
character, that occur in the strata intermediate between the Carbo- 
naceous and Silurian systems in the west of England and on the 
borders of Wales; and justly infers the inadequacy of any one term 
to characterize formations which vary so much in lithological com- 
position, that at one place they present the condition of a fine- 
grained silky slate, at another of sandstone, and at a third that of 
coarse gravel and conglomerate rock. 
Thus, with respect to the slate rocks of Devon, Cornwall and 
Wales, the difficulties are reduced to those of an unsettled nomen- 
