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654 STRATIGRAPHICAL GEOLOGY. [Boox VI. _ 
base of the great Arenig group or Lower Llandeilo rocks of Murchison. 
All his Silurian strata of older date than these rocks will be classed as 
Cambrian. It is undoubtedly true, however, that although a decided 
break occurs in the succession of species at the top of the Cambrian 
series, the general paleontological resemblance of the Cambrian and 
Silurian systems as thus discriminated is singularly close; so close 
indeed, that there may not improbably be a subsequent revision of this 
question with the result of throwing all these older Paleozoic rocks 
-into one paleontological system. . 
According to the classification here adopted, the Cambrian system, as 
developed in North Wales and the border English counties, consists of 
purple, reddish-grey, and green slates, grits, sandstones, and conglomer- 
ates, which are estimated to reach the enormous thickness of 25,000 feet. 
By far the larger part of this vast depth of rock is unfossiliferous. 
Indeed it is only in some bands of the upper 6000 feet, or thereabouts, 
that fossils occur plentifully. The total British Cambrian fauna dis- 
covered up to the present time embraces 61 genera and 182 species. By 
fossil evidence the Cambrian system may be divided into Lower and 
Upper, and each of these sections may be further subdivided into two 
groups, as in the following table : 
Cambrian 
3. Lingula flags. 
of Wales. 
Bower, a Menevian group. 
iis a Tremadoc slates. 
1. Harlech and Longmynd group. 
1. Harlech and Longmynd Group.—This group consists of purple, red, 
and grey flags, sandstones, and slates, with conglomerates. These strata 
attain a great thickness, estimated at 4000 feet in South Wales, more 
than 8000 in North Wales, and perhaps 25,000 in Shropshire. They 
were formerly supposed to be nearly barren of organic remains; but . 
in recent years, chiefly through the researches of Dr. Hicks at St. 
David’s, they have yielded a tolerably abundant fauna, consisting of 
32 species. Among these are 7 genera and 14 species of trilobites 
(Paradoxides, Plutonia, Microdiscus, Paleeopyge, Agnostus, Conocoryphe), 
four annelides (Arenicolites), a sponge (Protospongia), six brachiopods 
(Discina, Lingulella), two pteropods (Theca), &c. Many of the surfaces 
of the strata in some parts of this group are marked with ripples, sun- 
cracks, and rain-pittings, as well as with trails of worms—indicative 
of shallow-water and shore-conditions of deposit. Twelve of the 32 
species, according to Mr, Etheridge, pass up into the Menevian group.} 
2. Menevian Group.—This subdivision has been proposed for a series 
of sandstones and shales, with dark-blue slates, flags, and grey grits, 
which are seen near St. David’s (Menevia), where they attain a depth of 
about 600 feet. They pass down conformably into the Harlech group, 
with which, as just stated, they are connected by 12 species in common. 
The Menevian beds have yielded 52 species of fossils, of which 19 pass 
up into the lower Lingula flags. Among them the trilobites are 
specially prominent, 12 genera and 32 species having been obtained 
from the Menevian beds, among which the genera Agnostus (7 species), 
Conocoryphe (7 species), and Paradowides (3 species) are specially cha- 
racteristic. Four species of sponges (Protospongia), three of which are 
' Q. J. Geol, Soc, xxxvii, (1881), President’s address, p. 41. 
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