68 STRATA. 
Approximate 
*GROUP—(Era). SVYSTEM—(Period). thickness of Strata. 
I. Archizoic. Cambrian. 
Ordovician. 
Silurian. 
70,000 feet. 
Il. Paleozoic. Devonian. 
Carboniferous. 
Permian. 
42,000 feet. 
III. Mesozoic. Triassic. 
Jurassic. 
Cretaceous. 
15,000 feet. 
IV. Cainozoic. Eocene. 
Miocene. 
Pliocene. 
3000 feet. 
SS OES ie 
VV. Anthropozoic, Pleistocene. 
Recent. } 600 feet. 
This enormous thickness of about 130,000 feet represents 
a vast duration of time and we can only compare one part 
with another. 
It has been estimated that at the present time the average 
rate of deposition may be taken as about 1 foot in 1500 
years. This would give us about 200,000,000 years, which 
with corresponding periods of elevation might be 400,000,000 
years. Such a calculation is really of practically no value as 
there are many factors which might easily multiply the 
figures. 
The Archizoic group have strata in many cases modified 
by heat and pressure and they are probably by no means 
the first strata. In other words, the origin of animals is 
antecedent to the Archizoic Era. Thus, the strata of this 
era show Arthropoda, Echinodermata, Mollusca and other 
phyla, all sharply differentiated as at the present day. 
The geological record does not, therefore, help very 
much in giving us the original ancestors of these phyla, but 
it forms a very important guide with regard to the higher 
animals. Thus, although fishes are found in the Silurian 
system the other five orders of Vertebrata only occur there- 
after. Hence there is always hope that the geological record 
may assist us in tracing the descent of the higher vertebrate 
* This table is taken from Heeckel’s ‘‘ History of Creation.” 
