Geological Aspects of Evolution 97 
of the Paleozoic era brought with it the rise of the Appalachian 
Mountains. It has been estimated that only three percent of the 
Paleozoic forms of life survived and lived on into the Mesozoic 
era. The ‘birth of the Rocky Mountains at the close of the 
Mesozoic was accompanied by widespread destruction of reptilian 
life. Some of the intervals of erosion were so long that mountains, 
miles high were eroded to a flat surface before sedimentation was 
resumed on the truncated edges of the mountain structure (See 
figure 1). ‘These intervals allowed time for the changes in life 
recorded in the overlying sediments. In figures 2 and 3 there are 
shown rocks belonging to each era of the earth’s history separated 
by erosion intervals.. The distances involved are so great that it 
is impossible to show in a single photograph the entire sequence. 
However, by carefully tracing the sequence of rocks of one region 
into another, it is possible to determine correctly the stratigraphic 
position of the formations and to construct a geologic cross- 
Section showing their structural relations. This has been done in 
figure 2 for the region of the Grand Canyon and the Colorado 
High Plateaus, in which nearly ninety-seven percent of the geo- 
logic record is represented. In the Mississippi Valley the rocks 
of each successively younger period overlap from north to south 
like shingles on a roof (Figure 3). 
‘To mention the appearance of each higher type of life with 
advancing time gives only part of the story. Each early repre- 
Sentative of a class was primitive and generally its span of life 
was short so that it can be used as a guide fossil to the period 
in which it lived. The highest developed fish of the Age of Fishes 
differed from any fish living today, in being covered with bony 
plates instead of scales and in the structure of its tail. The primi- 
tive amphibian, Eryops, is limited to the Upper Paleozoic. A 
Stegocephalian Amphibian was the first animal to possess a skel- 
¢ton having four limbs, each terminating in five digits. This skel- 
etal plan, which is common to all the higher vertebrates, originated 
in Upper Paleozoic time. During the Age of Reptiles the dino- 
Saurs and related forms dominated the land, the sea and the air, 
but all of these including the largest land animal that ever lived, 
