60 
BRITISH ASSOCIATION.- 1835 . 
the “ Caradoc” and “ Llandeilo” the “ Lower Silurian rocks'. He 
further remarked, that in South Wales he had traced many distinct 
passages from the lowest member of the “ Silurian system” into the 
underlying slaty rocks, now named by Professor Sedgwick the 
“ Upper Cambrian .” 
This communication was illustrated by Ordnance Maps extending 
over large parts of eleven counties, coloured in the field by Mr. Mur¬ 
chison. 
Professor Sedgwick commenced by pointing out the imperfection 
of the sections exhibited in the North of England, and some por¬ 
tions of North Wales, in consequence of the entire want of conti¬ 
nuity between the carboniferous series and the inferior schistose 
groups. Some of the latter are fossiliferous both in Denbigh¬ 
shire and Westmorland; but in the interrupted sections of those 
counties it is impossible to tell how many terms are wanting to com¬ 
plete the series to the old red sandstone and carboniferous lime¬ 
stone. In the country described by Mr. Murchison these difficulties 
do not exist, and his sections have filled up a w ide chasm in the 
succession of British deposits. Professor Sedgwick then described 
in descending order the groups of slate rocks, as they are seen in 
Wales and Cumberland. To the highest he gave the name of Upper 
Cambrian group. It occupies the greatest part of the chain of the 
Benvyns, w T here it is connected with the Llandeilo flags of the Silu¬ 
rian system, and is thence expanded through a considerable portion 
of South Wales. In one part of its course it is based on beds of lime¬ 
stone and calcareous slate; but on the w r hole, it contains much less 
calcareous matter than the Silurian system, and has fewer organic re¬ 
mains. Beds of good roofing-slate occur, and a perfect slaty cleav¬ 
age is often observed in it transverse to the stratification; but other 
parts of it are of a coarse mechanical texture. To the next inferior 
group he gave the name of Middle Cambrian. It composes all the 
higher mountains of Caernarvonshire and Merionethshire, and 
abounds in fine roofing-slate, alternating with, and apparently pass¬ 
ing into, irregularly interstratified masses of porphyry. Some por¬ 
tions of it are coarse and mechanical, and it contains (for example, 
at the top of Snow r don,) a few organic remains, and a few examples 
of highly calcareous slates, but no continuous beds of limestone. 
The same group, with the same mineral structure, and in the same 
position, but without organic remains, is greatly developed in Cum¬ 
berland. The Lower Cambrian group occupies the S. W. coast of 
Caernarvonshire, and a considerable portion of Anglesea: it consists 
chiefly of chlorite schist, passing here and there into mica schist and 
slaty quartz rock, and contains subordinate masses of serpentine and 
white granular limestone. It contains no organic remains. Beneath 
the Middle Cambrian system (above described) there occurs in 
Cumberland (for example, Skiddaw Forest,) a great formation of 
dark glossy clay slate, without calcareous matter, and without organic 
remains. It passes in descending order into chiastolite slate, mica 
slate, hornblende slate, gneiss, &c., w-hich rest immediately on gra¬ 
nite. Whether the Lower Cambrian was to be placed on the exact 
