140 
THE CAMBRIAN VOLCANOES 
BOOK III 
able, that many geological forces may have been more vigorous in primeval 
times than they afterwards became. Bnt of the gigantic tides, prodigious 
denudation and Auolent huddling together of the waste of the earth’s 
surface, which have been postulated for the early Palteozoic ages, there is 
assuredly nowhere any indication among the stratified formations. In 
those vast orderly repositories, layer succeeds layer among thinly -lami- 
nated shales, as gently and equably as the fine silt of each tide sinks to- 
day over the floor of a sheltered estuary. At the primeval period of 
which these sediments are the memoiial, the winters receded from flat 
shores and left tracts of mud bare to the sky, precisely as they do still. 
Then as now, the sun shone and dried such mud-flats, covering their surfaces 
with a network of cracks ; the rain fell in heavy drops, that left their 
imprints on the drying mud; and the next tide rose so gently as to over- 
flow these records of sunshine and shower without eflacing them, but 
spreading over them a fresh film of sediment, to be succeeded by other 
slowly-accumulating layers, under which they have lain preserved during 
the long cycles of geological history. 
That organized creatures had already appeared upon the earth’s surface 
before tlie beginning of the Cambrian period cannot be doubted. The 
animal remains in the lowest Cambrian strata are far from being the simple 
forms which might be expected to indicate the first start of animal life 
upon tlie surface of the earth. On the contrary, though they are com- 
paratively scanty in types, and often rare or absent tliroughout a thick 
mass of sedimentary deposits, tliey show beyond dispute that, when they 
llomished, invertebrate life had abeady reached such a stage of advancement 
and differentiation that various leading typies had appeared which have 
descended, in some cases with generic identity, down to our own day. There 
must have been a long pedigree to these organisms of the oldest known 
fossiliferous rocks. And somewhere on the earth’s surface we may yet hope 
to find the remains of their progenitors in pre-Cambrian deposits. 
The researches of many explorers in Europe and Xorth America have 
brought to light an interesting series of organic remains from the Cam- 
brian system. Of the plants of the thne hardly any traces have survived, 
save some markings which have been referred to sea-weeds. The earliest 
known sponges and corals occur in this system, likewise the ancestors of 
the graptolites, which played so prominent a part in the life of the next or 
Silurian period. There were already representatives of crinoids and star- 
fishes, besides examples of the extinct group of cystideans. Sea-worms 
crawled over the muddy and sandy sea-bottom, for they have left their 
trails and burrows in the hardened sediments. Molluscs had by this time 
appeared in their four great divisions of Brachiopods, Lamellibranchs, Gastero- 
pods and Cephalopods, though the forms yet discovered among Cambrian 
rocks are comparatively few. The most abundant and characteristic inhabit- 
ants of the Cambrian seas were the trilobites, of which many genera have 
been disinterred from the strata. In the lowest fossiliferous Cambrian 
group the trilobitic genus Olenellus, already referred to, is the characteristic 
