114 HISTORICAL GEOLOGY 



crust and thus are of the highest value and interest to the student of 

 historical geology. Since, however, the remains of only a small propor- 

 tion of the animals living at a given period are permanently preserved 

 in the marine, river, lake and subaerial deposits of that period, the 

 geological record of animal and plant forms is far from complete. Inas- 

 much as invertebrate animals are far less free in their movements than 

 the vertebrate forms, they are accepted as the best determinants of the 

 geological age of a bed of rock, even when remains of both kinds are 

 found together. Invertebrate life, too, appeared on the globe far earlier 

 than vertebrate, and remains of certain species are abundant in the 

 lowest, "oldest," of our stratified rocks. 



At the left near the entrance to the hall there has been installed a 

 topogeological or relief map model of the Bright Angel section of the 

 Grand Grand Canyon of the Colorado. The scale is large enough 

 Canyon to give the visitor a vivid idea of the extensive erosion 

 Model {fiat has taken place in a famous region where the 

 geology lies spread out so plainly that he who runs may read. This is the 

 first of a series of models which have been planned to occupy the ends 

 of the upright cases throughout the hall, illustrating the most evident 

 and striking results of the action of geological forces. 



In the desk cases down the center of the hall are about 8350 type and 

 T figured specimens used by James Hall, R. P. Whitfield 



and others in the original description and naming of 

 species, or in their further elucidation. 



The specimens in the cases on the left or west side of the hall are being 

 arranged to illustrate stratigraphic geology, beginning at the south 

 Stratigraphic (entrance) with the Archean rocks, which are the lowest 

 or Historical and oldest of all and contain no fossils, advancing regularly 

 Geology through the Cambrian, Ordovician, Silurian, Devonian, 



Carboniferous, Jurassic, Triassic, Cretaceous and Tertiary. Most of the 

 specimens on exhibition are from American localities and the species 

 are arranged according to their position in the scale of life, the lower, 

 or simpler forms being placed first. The specimens shown are those 

 particularly characteristic of the various horizons, the object being to 

 give an idea of the general character of the life of different periods of the 

 world's history. 



The specimens on the east, or right, side are being arranged to illus- 

 trate biologic geology, the classification and relationship of the plants 

 Biologic and animals of past geologic times. The series starts 



Geology with the plants and is followed by the various subdivisions 



of the animal kingdom, again beginning with the lower, or simpler forms 

 and continuing to the highest. 



