160 PAST AND PRESENT 
—w) 
us any insight into the evolution of the earlier forms 
of life. Below the Cambrian rocks, as represented in 
these islands and in Europe generally, with their well- 
developed fauna, are tens of thousands of feet of 
strata which once, no doubt, were sediments at the 
bottom of the sea, and later on hardened into slates 
and sandstones in which were embedded remains of 
more primitive organisms; but these rocks have been 
so altered during the immense period of their exist- 
ence by heat and pressure and the other vicissitudes 
to which the restless crust of the earth is subject that 
they now present a mass of granite-like material in 
which all trace of organic life has been destroyed. 
In America the rocks of corresponding age are better 
preserved, and have yielded a limited fauna display- 
ing an already advanced stage of evolution. To 
account for the strange paucity of animal remains it 
has been suggested that the creatures of these earliest 
times were soft-bodied, so that after death they left 
no trace behind. It may be noted that the pre-Cam- 
brian rocks contain beds of limestone and of carbon 
(in the form of graphite); such beds, in later rocks, 
are composed of organic materials, the limestones 
being formed of the skeletons of minute marine 
creatures, particularly Foraminifera, and the carbon 
deposits of the remains of plants. 
In Cambrian times, then, abundant life springs forth 
into our vision from the rocks, already, like Minerva, 
fully armed. The soft plant structures are not well 
preserved in the older fossiliferous rocks, and hence 
the fragmentary story of plant life, as we trace it 
backwards, becomes very obscure, while many types 
of animals still boldly occupy the stage. At the 
