CLASSIFICATION OF PALAEOZOIC ROCKS OF THE BRITISH ISLES. 299 
finer material to the former, and tlie dark colour to carbon 
and other substances derived from tbe latter. Though not of 
great thickness, the group is yet divisible into three well-marked 
zones, with distinct species in each. The genus Paradoxides, 
of which so many species occur in the Lower Cambrian rocks, 
does not in this country reach into higher beds than this group ; 
and by it as well as by several other forms ( Anopolenus , &c.) 
the Lower Cambrian is palaeontologically well marked off from 
the Upper Cambrian. There is also an indication of some slight 
physical change at the close of the Menevian, in the sudden 
appearance of thick grit beds upon very fine muddy deposits. 
This is partially shown in the view (PL YlI.) taken of the east 
side of Porth-y-rhaw, the place where the group was first 
made out. Whether this change had anything to do with the 
disappearance of Paradoxides and its allied genera or not, is 
difficult to make out. It was, at any rate, sufficiently 
important to have produced identical results in North Wales 
at the same time, for the upper beds are there, as in South 
Wales, capped by these massive sandstone beds. This group 
is everywhere distinct from, and should not be included in, 
the so-called Lower Lingula flags.'* The fauna belonging to 
this group consists of twenty-three genera, including fifty- two 
species, and nearly all the species are restricted to it ; the 
range of many is limited to a few beds. This group has been 
discovered at several places in Merionethshire, and contains 
there most of the characteristic fossils found by us at St. 
David’s. It is there seen to rest on the well-known Harlech 
grits, and to be succeeded by Lower Lingula flags. 
| Sandstones and Shales, with Orthis Hicksii, &c. 
| Flags and Slates, with Paradoxides Davidis , &c. 
| Grey Flags, with Paradoxides Hicksii, &c. 
The views, Pis. YII., andYIII., figs. 1-3, show tolerably well 
the succession, and some of the chief sections of the Lower Cam- 
brian rocks on the north shore of St. Bride’s Bay, near St. David’s. 
The Upper Cambrian consists of four groups. The three 
lower ones were carefully described by the late Mr. Belt in 
1867, f and named by him, in ascending order, Maentwrog, 
Pfestiniog, and Dolgelly groups (/, g, h, in sections and plates). 
* It was by mistake that Mr. Etheridge, in the admirable address already 
referred to, included Paradoxides and some other Menevian genera in the 
Lower and Upper Lingula flag faunas. 
t Geol. Mag. 1867 and 1868. 
M Ph 
> 
m o 
fc « 
3 ® 
Upper, 
100 ft. 
Middle, 
350 ft. 
Lower, 
300 ft. 
