The Dinosaurs of Wyoming - Wyoming State Geological Survey ...

FIG. I.-DIAGRAM ILLUSTRATING NORTH AMERICAN HISTORICAL
GEOLOGY
The Mesozoic Era, the Age of Reptiles, which witnessed the life of the dinosaurs,
is near the top.
The rivers on the earth have always carried mud.> sand and gravel to the sea, which,
in settling, have spread out in layers over the sea bottoms or in lakes, rivers and
swamps. Remains of various forms of life, such as shells and bones, accumulated
after death in these layers, where the hard parts were preserved as fossils. In time,
these sediments consolidated into hard rock and have been elevated above the sea level.
The geologist studies these ancient deposits, which now form a large part of the
earth's surface, and from the nature of the sediments and from the life remains or
fossils, that they contain, he is able to reconstnlct much of the past history of the
,arth. The rocks in the earth's crust give evidence also of the physical conditions under
which they were formed, and apparently the physical processes, such as erosion and
weathering, have not changed throughout all geological time. The life on the earth,
however, is constantly varying, ·owing to change of environment, and species after
;,pecies sooner or later die out to be replaced by other forms of life. Rocks of similar
age therefore contain similar species of fossils. Dinosaur history occupied only a small
part of geologic history, although existing for millions of years.
If all the sedimentary rocks of past ages had been accumulated in their greatest
thickness at one place they would form a succession of strata over 40 miles in height.
-After Dr. R. S. Bassler.
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