528 REPOET OF NATIONAL MUSEUM, 1890. 



Jasper is a dull or bright red or yellow variety of silica containing 

 alumina, and owing its color to iron oxides. Characteristic forms are 

 shown in specimens from Colorado and Montana (10374, 10397, and 

 38572). 



Chalcedony is a translucent, massive variety occurring mainly in 

 cavities in older rocks where it has been deposited by infiltration. The 

 specimens from Montana (38G05) and Cuba (3(3140) are common forms. 

 No. 70064 from Webster, Jackson County, North Carolina, is a cellular 

 variety formed in seams in dunite rocks during the decomposition of 

 the olivine; No. 36051 and 36140 are somewhat similar varieties from 

 Sebastopol, Georgia, and Jonarre, France, used for buhrstones in 

 grinding grain. Specimens 35599 and 36010 from Wyoming and Colo- 

 rado are silicifled woods partaking of the character of chalcedony rather 

 than opal, as in the cases above mentioned. 



Flint is a variety of chalcedony formed by segregation in chalky lime- 

 stones, and composed in part of the broken and partially dissolved 

 spicules of sponges and the remains of infusoria. Chert is an impure 

 flint containing frequently fossil nummulitic remains (26581), and with 

 sometimes an oolitic structure ; oolitic varieties, such as that from Cen- 

 tre County, Pennsylvania (70606) are not common. The variety flint 

 is shown by a characteristic sample from the English chalk beds 

 (36012). The cherts are illustrated by specimens from Kansas (26581); 

 Missouri (17598); Texas (70429); Nevada (21762), and Pennsylvania 

 (70127). 



The name novaculite is frequently given to very fine grained and 

 compact quartz rocks, such as are suitable for hones. As commonly 

 used the name is made to include rocks of widely different origin, some 

 of which are evidently chemical precipitates, while others are indurated 

 clastic or schistose rocks. Here are placed the well known "novacu- 

 lites" of Arkansas, which are considered by authorities to be altered 

 cherts (specimens 4307, 27833, and 39109). 



Quartz is a massive form of crystalline silica occurring in veins, dis- 

 seminated granules, and pockets in rocks of all kinds and all ages. It 

 is often colored pink or reddish by iron oxides. Many other varieties 

 of silica occur, but are not sufficiently abundant to constitute rock 

 masses, and are to be found in the collection of the mineral department. 

 It is represented here by specimens from Auburn, Maine (37613) ; Bed- 

 ford, New York (36058) ; Lake Superior, Michigan (4260) ; Sawatch 

 Mountains, Colorado (35896); Godhaveu, Greenland (34947); Brazil 

 (4092), and Freiberg, Saxony (3836). 



Lydian stone is an exceedingly hard impure quartz rock of a black 

 color and splintery fracture. It was formerly much used in testing the 

 purity of precious metals. (Specimen 3820 from Frankenberg, Saxony.) 



(2) Carbonates. — Water carrying small amounts of carbonic acid 

 readily dissolves the calcium carbonate of rocks with which it comes 

 in contact, taking it up in the form of bicarbonate ; on evaporation this 



