GEOLOGY IN NATIONAL MUSEUM — MERRILL. 299 



grees of purity, which are utilized in their natural conditions, or 

 from which materials for paints can be prepared. These include 

 the iron oxides of prevailing yellow or brown colors, though 

 sometimes red ; chromite, used in the manufacture of chrome, yellows 

 and greens ; barite or sulphate of barium, used as a substitute for white 

 lead; and a few other minerals utilized to some extent for similar 

 purposes. These are shown in their natural conditions as well as 

 ground, purified, and otherwise prepared for use. 



Graphite, plumbago, or black lead as it is sometimes popularly 

 called, is shown in an upright case in all its principal varieties as 

 disseminated in the rock, in clean crystalline lamellar masses like 

 those of Ceylon or in impure and amorphous forms, as those of 

 Sonora, Mexico. A series of jars show the pure Ceylon material as 

 crushed and prepared for use. 



The coals, in a similar case, are shown in all phases of carboniza- 

 tion from peat as freshly taken from a bog, through the lignitic or 

 brown coal and bituminous phases to anthracitic and graphitic forms 

 in which metamorphism has progressed so far as to render the ma- 

 terial unsuited to fuel purposes. 



The kaolins, clays, and fictile material in general occupy two Amer- 

 ican cases at the west end of the hall. The series are selected to show 

 varieties rather than distribution. They include the kaolins, both 

 crude and washed ready for use in the higher grades of china and 

 porcelain ware, the less pure forms used in pottery and the coarser 

 admixtures utilized for brick tile and terra cotta. While it is rec- 

 ognized that as soon as material passes from the hand of the miner 

 to that of the metallurgist, chemist, or assayer it leaves the domain of 

 applied geology and enters that of technology, it has nevertheless 

 been deemed advisable with many of the nonmetallic minerals, rare 

 earths and metals to include enough of the manufactured product to 

 suggest their possibilities. In the case of some of the metals this 

 has been already referred to (p. 297). Such treatment seems espe- 

 cially desirable in the case of the clays where a possible change in 

 color during firing may be an important matter. Hence in these 

 same cases samples of the raw and baked and partially finished prod- 

 uct are exhibited side by side. 



On the south side of the hall are shown materials used for struc- 

 tural and monumental purposes, i. e., building stones and marbles. 

 This collection was begun and a considerable part of it was made 

 under the direction of Dr. George W. Hawes while in charge of the 

 geological collections and acting as a special agent of the Tenth 

 Census. The stones are in large part cut into the form of 4-inch 

 cubes for convenience of display and installation. Against the walls 

 and as panels of special bases are large slabs selected to show vena- 



