276 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



and mammoth. Other cases are devoted to the Tertiary life of the 

 continent. The fossil rhinoceroses are represented by numerous 

 skulls and jaws and two mounted skeletons. The short-legged form 

 (Teleoceras fossiger) from Kansas and Diceratherium cooki from 

 western Nebraska are notable examples of their kind. 



The collection of Titanotherium remains is probably the most 

 ambitious display of this great group of extinct mammals ever at- 

 tempted. Twenty-two selected skulls, representing G genera and 

 20 species, together with a mounted skeleton of Brontotherium 

 hatcheri makes an exhibit of exceptional merit (pi. 8, fig. 2). 



DIVISION OF PHYSICAL AND CHEMICAL GEOLOGY. 

 SECTION OF SYSTEMATIC GEOLOGY, EAST RANGE, FIRST FLOOR. 



In this division attention is given to the bringing together and, 

 so far as is practicable, preparing an exhibit of materials bearing 

 upon the history of the earth in its cosmical aspect and illustrative of 

 its composition and structure. It begins with a series, showing so 

 far as their nature will permit, (1) the chemical elements consti- 

 tuting rocks, (2) the minerals constituting rocks, (3) the physical 

 properties of rocks as structure, color and fracture, and (4) the 

 kinds of rocks (Petrology). (See pi. 10, fig. 2.) 



(1) Chemical elements. — Although there are some 80 known ele- 

 ments, but 16 occur in o.r\y great abundance or form more than an 

 extremely small portion of the earth's crust. These 16, exhibited 

 and arranged according to their chemical properties and in the order 

 of their abundance, are as follows: 



Metalloids: 

 Oxygen. 

 Silicon. 

 Carbon. 

 Sulphur. 

 Hydrogen. 

 Chlorine. 

 Phosphorus. 

 Fluorine. 



Metals : 



Aluminum. 



Calcium. 



Magnesium. 



Potassium. 



Sodium. 



Iron. 



Manganese. 



Barium. 



(2) The minerals constituting rocks. — This series represents the 

 various compounds of the elements mentioned above in the i'orm 

 of recognized mineral types, only such being shown as are com- 

 monly found in the form of mineral aggregates to which the term 

 rock is applied. As exhibited they are divided into primary, sec- 

 ondary, and accessory, according as they are products of the first 

 consolidation of the rock mass, or clue to secondary changes, or are 

 accidental constituents. 



(3) The Physical and Chemical Properties of Rocks. — Selected 

 specimens illustrating structure — the manner in which their various 



