CHAP. IX 
THE CAMBRIAN SYSTEM OF BRITAIN 
41 
form. Higher up Paradoxides is predominant, while towards the top of the 
system the most characteristic genus is Olenus. 
From the organic remains which have been preserved, we may legiti- 
mately infer the existence of others which have entirely disappeared. There 
seems no reason to doubt that the leading grades of invertebrate life which are 
wanting in the known Cambrian fauna were really represented in the Cam- 
brian seas. The chance discovery of a Ijand of limestone may any day 
entirely alter our knowledge as to the relative proportions of the several 
divisions of the animal kingdom in the earliest Palseoxoic rocks. Sand 
is rather adverse to the preservation of a varied representation of the 
organisms of the overlying sea- water. Mud is generally favourable, but cal- 
careous accumulations are greatly more so, and they usually consist almost 
entirely of organic remains. Thus in the Cambrian series of the north-west 
of Scotland the quartzites that form the lower group, though sometimes 
crowded with worm-burrows, contain hardly any other sign of organisms. 
The overlying shales, besides their abundant worm-castings, have yielded 
perfect specimens of Olenellus <and other fossils. But in the uppermost group, 
consisting of limestones, every particle of the sediment appears to have passed 
through the intestines of worms, and as it gathered on the sea-bottom it 
enclosed and has preserved a varied and abundant assemblage of organisms, 
including trilobites, gasteropods and a number of cephalopods. MTiile in 
the Cambrian rocks of Europe calcareous bands are comparatively rare, in 
those of North America they are not infrequent. Hence it is largely from 
American deposits that our knowledge of the Cambrian fauna has been 
derived. 
Not a vestige of any vertelirate organism has yet been detected among 
the earlier Pahnozoic sediments. So far as we know, there were no fishes 
in the Cambrian seas. The highest organisms then existing were cham- 
bered shells, a once abundant and singularly varied class, of which the living 
Nautilus is now the sole representative. 
In trying to realize the general geographical conditions of Cambrian 
time, the geologist finds himself entirely without any evidence as to the 
character of the terrestrial vegetation. We can hardly doubt that the land 
was clothed with plants, probably including lycopods and ferns, possibly 
even cycads and conifers. But no remains of this flora have yet been 
recovered. Nor have any traces of land-animals been detected. All that 
we yet know of the life of the period has been gleaned from marine sedi- 
nients, which show that the invertelirate population by which the sea was 
then tenanted embraced some of the leading types of structure that have 
survived through all the long vista of geological time down to our own 
day. 
Some of the shore-lines of the Cambrian waters may still be traced, and 
it is possible to say where the land of the time stood and where lay the sea. 
In the British area the largest relic of Cambrian land is found in the far 
north-west of Scotland. Formed partly of the Lewisian Gneiss and partly 
of the Torridon Sandstone, it takes in the whole chain of the Outer 
