18 A FIRST CHAPTER 
being that they are covered by water, or at least buried in wet ground. 
Later through subsequent upheaval and erosion they are again brought 
to or near the surface of the earth. Petrifaction is the slow replace- 
ment of animal or vegetable material by such minerals as carbonate of 
lime or silica. The process is very slow and for this reason flesh is never 
petrified. Fossil beds are found in every continent. In our own coun- 
try, Texas, Montana, Wyoming, and the Bad Lands of South Dakota 
are famous for their large fossil beds, and many of the finest and rarest 
fossils in the Museum were obtained in these localities. 
As it takes thousands of years for the various layers of earth to 
accumulate over the bones, and for the latter to become petrified, the 
study of fossils and the strata in which they are found is an important 
aid in determining the age of the earth and the succession of life thereon. 
The Divisions of Geologic Time are based on the character of their 
life as indicated by fossils. 
The estimated Duration of Geologic Time is based largely on the 
thickness of the rocks. 
Exhibits relating to the geologic history of the earth, and its 
past life, will be found on the fourth floor of the Museum. Those in 
the Hall of Geology illustrate the structure of the earth, include 
examples of the various rocks of which it is composed, and are 
accompanied by fossils showing the general character of the life of the 
different geologic periods and the steps or stages that have led to the 
animal life of the present. 
Proceeding to the Southeast Pavilion and going westward, one 
passes from the hall devoted to fossil fishes, through those containing 
reptiles and mammals, coming finally to the Hall of the Age of Man, 
and in afew minutes witnesses changes that required millions of years for 
their accomplishment. The strange armor-clad fishes, unlike any now 
living, were succeeded by amphibians and reptiles, among them curious 
creatures like Naosaurus and the great dinosaurs which in their day 
were the rulers of the earth. Here are huge herbivorous reptiles like 
Brontosaurus and Triceratops and the flesh-eating creatures that preyed 
upon them, including Tyrannosaurus, the most formidable beast of 
prey that ever lived. They passed out of existence and gave place 
to the mammals, and these underwent many changes before the forms 
of to-day appeared. Lastly came man, a weakling compared with the 
animals by which he was surrounded, which include such forms as the 
mammoth, mastodon and great ground sloths, and yet, by reason of his 
superior brain and its servant the hand, coming to dominate them all. 
Many of the animals shown in these halls are described in Hand- 
book Animals of the Past, another is devoted to Dinosaurs, and leaflets . 
discuss Mammoths and Mastodons and the Evolution of the Horse. 
