THE CAMBRIAN PERIOD. Th 
CHAPTER “Vile. 
THE CAMBRIAN PERIOD. 
The traces of life in the Laurentian period, as we have seen, 
are but scanty ; but the Cambrian Rocks—so called from their 
occurrence in North Wales and its borders (“‘ Cambria”)—have 
yielded numerous remains of animals and some dubious plants. 
The Cambrian deposits have thus a special interest as being 
the oldest rocks in which occur any number of well-preserved 
and unquestionable organisms. We have here the remains of 
the first fauna, or assemblage of animals, of which we have at 
present knowledge. As regards their geographical distribu- 
tion, the Cambrian Rocks have been recognised in many parts 
of the world, but there is some question as to the precise limits 
of the formation, and we may consider that their most typical 
area is in South Wales, where they have been carefully worked 
out, chiefly by Dr Henry Hicks. In this region, in the neigh- 
bourhood of the promontory of St David’s, the Cambrian Rocks 
are largely developed, resting upon an ancient ridge of Pre- 
Cambrian (Laurentian?) strata, and overlaid by the lowest 
beds of the Lower Silurian. ‘The subjoined sketch-section 
(fig. 27) exhibits in a general manner the succession of strata 
in this locality. 
From this section it will be seen that the Cambrian 
Rocks in Wales are divided in the first place into a lower and 
an upper group. The Zower Cambrian is constituted at the 
base by a great series of grits, sandstones, conglomerates, and 
slates, which are known as the “‘ Longmynd group,” from their 
vast development in the Longmynd Hills in Shropshire, and 
which attain in North Wales a thickness of 8000 feet or more. 
The Longmynd beds are succeeded by the so-called “‘ Mene- 
vian group,” a series of sandstones, flags, and grits, about 600 
feet in thickness, and containing a considerable number of 
fossils. ‘The Upper Cambrian series consists in its lower por- 
tion of nearly 5000 feet of strata, principally shaly and slaty, 
which are known as the “Lingula Flags,” from the great 
abundance in them of a shell referable to the genus Zzzgu/a. 
These are followed by tooo feet of dark shales and flaggy 
sandstones, which are known as the “‘ Tremadoc slates,” from 
their occurrence near Tremadoc in North Wales ; and these 
in turn are surmounted, apparently quite conformably, by the 
basement beds of the Lower Silurian. 
