522 



REPORT OF NATIONAL MUSEUM, 1900. 



Quartz iiu-ludes a larger number of kinds of gems among its varie- 

 ties than any other mineral. The varieties arise from differences in 

 color, mode of formation, or (nystallization. These varieties, though 

 differing wideh' in appearance and passing uifder a variety of names, 

 are chemically all of one sul)stance. The following is a list of the 

 kinds commonly recognized: 



Agate. — A variegated chalcedonic quartz, the colors in clouds, spots, 

 bends, or layers. The bands or layers may be either parallel or concen- 



Fig. 9. 



AGATIZED WOOD. 



Chalcedony Park, Arizona. 

 .Specimen No. 82485, U.S.N. M. 



trie, and either in straight, circular, or zigzag forms. Banded agates 

 may be called fortification^ handed., or eye agates^ according to the 

 arrangement of the layers, (See fig. 8.) 



Agatized 'wood.— \\oo([ siliciried by silica and usually having the 

 structure of the original material. The western part of the United 

 States affords some remarkal)le specimens of this material. In Apache 

 County, Arizona, there is a silicitied forest in which trunks 150 feet 



