Subdivisions of the Paleozoic Strata of Great Britain. 343 
matter undergoing change, would impart to the water of the ad- 
joining strata the power of dissolving the diffused proto-carbonate, 
which, being then carried by infiltration through the more porous 
beds, would accumulate above and within the close argillaceous 
or shaly layers, forming in some cases bands of rock ore, in 
Others courses of nodular and plate ores. Of these, the former 
would seem to have resulted from the accumulation by gravity of 
the dissolved carbonate in the substance of sandy shales near the 
upper limit of the more impervious beds, while we may regard 
the latter as having been collected in all directions from the gen- 
eral charge of proto-carbonate accumulated in the argillaceons 
Mass, its mobility in the dissolved condition greatly aiding the 
gathering process of the segreguting force. 
Arr. XXXIV.—Subdivisions of the Paleozoic Strata of Great 
Britain, according to Prof. Sedgwick.* 
I. Lower Paleozoic Division, representing the Cambrian and 
Silurian Series in ascending groups. 
( a. Longmynd slates, ce. 
Llanberris slates ; alternations of roofing- 
1. Longmynd and Bangor group slates and grits. 
(Lower Cambrian) c. Harlech grits; sometimes approaching a 
L conglomerate form. 
2. F estiniog group 
( ve 
c. 
; ; Slates, flags 
Middle Cambrian) alternations of porphyry and trap-shale: 
one i a one near th 
a 
A gr 
atl Ara to the Bala limestone. __ 
b. Upper Bala rocks. er this term are 
Oambrian Series. 
5 
HY : 
28 S65 
8 
B 
& 
z 
3. Bala group : 
i —(2) Flag-stones, slates, calcareous 
(Upper Cambrian) © flag (2) ee ae 
oc. The w 
North Wales, with slate and tone ; 
and in South Wales, with slates, grits, 
conglomerates. The group 
coarse 
of very great 
Immediately above these three groups there isa great change 
of physical éonditions. ‘The most characteristic aud abundant 
* Extracted grea British Paleozoic Rocks, by the Rev. 
Ader aie M. ae ps So and ‘ba Bete Palwozoic Fossils, by Professor 
Frederick M Coy. F.GS &e., 4to, London, 1855, briefly noticed at page 302 of this 
volume, Mae 2 7. > 
