Geological Aspects of Evolution 93 
or thrust faulting. The rocks of the accessible part of the earth 
are divided into five major divisions or eras, which have been 
determined and named on the basis of the fossils contained in 
them (See table 1). The oldest rocks belong to the Archeozoic 
€ra, meaning the era of beginning of life. Archeozoic rocks are 
overlain by those of the Proterozoic era of early life and these 
may be traced upwards into rocks of the Paleozoic era of ancient 
life. Upon the Paleozoic strata rest the rocks of the Mesozoic era 
of intermediate life. At the top of the column are the rocks of the 
Cenozoic era of recent life. The eras are divided into periods 
which may be grouped together to indicate the highest forms of 
life during that interval. The nature of the first life preserved 
as fossils has long interested geologists. And yet uncertainty 
surrounds the earliest form, because Archeozoic rocks have every- 
where been intensely altered by heat, pressure, and earth move- 
ment and because very likely the earliest forms of life were with- 
out hard parts and unsuited to preservation. A discovery in the 
Archeozoic of Minnesota has been described as blue-green algae, 
a unicelled plant. Everyone does not agree on the evidence, but 
in the Proterozoic rocks there is undisputed evidence of algae 
and animal life, of which the highest form has been interpreted 
as a fragment of a crustacean, an invertebrate belonging to the 
phylum, Arthropoda. In the lower Paleozoic, fossils belonging to 
every phylum except the Chordata are prolific, so that this period 
may be referred to as the Age of Invertebrates. As increasingly 
younger rocks are examined, higher types of life appear in the 
Proper order, fish, amphibians, reptiles, mammals, man. This is 
Suggestive of the evolution of more highly developed types with 
the passage of time. The geologist is frequently asked how many 
years have passed since certain forms first appeared. In an en- 
deavor to answer these questions, the most reliable data available 
have been assembled as Table 1. The beginning of the Paleozoic 
era when only invertebrates lived is placed at five-hundred million 
years ago, the beginning of the Mesozoic era or Age of Reptiles 
at one-hundred-seventy million years ago, the beginning of the 
Tertiary or Age of Mammals at sixty million years ago, and the 
beginning of the Quaternary or Age of Man at one million 
