298 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



minerals are distinguished by their softness and consequent lack of 

 grit. They differ in that the first named is a hydrosilicate of mag- 

 nesium while the second is a hydrosilicate of aluminum. They are 

 almost equally soft and used for similar purposes, although perhaps 

 the pyrophyllite is the less common of the two. They are ground 

 and used for talcum powders and lubricants and the massive forms 

 as tailor's chalk, and in the manufacture of gas nipples and electrical 

 goods. The massive impure form, soapstone, is used for laundry 

 and laboratory tanks as well. Succeeding this is a case of micas, 

 under which name is included a number of minerals distinguished 

 by their eminent cleavage which permit of their being split into 

 transparent or translucent more or less elastic foliae. Of all the 

 varieties known, only the white mica muscovite and the pearl-gray 

 phlogopite have at present any commercial value. They are utilized 

 mainly in electrical work, though large quantities are ground and 

 utilized in the manufacture of wall paper and for producing frost 

 effects on Christmas cards, etc. 



Cases containing the haloid salts (fluorite, cryolite and common 

 salt), the nitrates, borates and sulphates, including a full series of 

 the Chilean nitrates and the potash salts from the celebrated deposits 

 at Stassfurt, Germany, follow in the order named. Two cases are 

 devoted to the so-called rare earths under which are included the 

 monazite sands and the compounds known under the mineralogical 

 names columbite, samarskite and zirkite — compounds of columbium, 

 cerium, yttrium, tantalum, etc., and the minerals lepidolite and 

 spodumene, which are possible sources of lithia. In continuation of 

 this series along the north wall on the north side of the hall are shown 

 in the order mentioned the minerals sepiolite (meerschaum), magne- 

 site, and dolomite, used in the manufacture of pipes and for refrac- 

 tory materials, or as sources of carbonic acid; limestones, used in 

 quick lime and for fluxes; sulphur and sulphur ores; gypsum, utilized 

 in the manufacture of land plaster, and plaster of Paris ; phosphates, 

 utilized for fertilizer purposes; abrasives, including the minerals 

 quartz, corundum, emery, volcanic dusts, diatomaceous earth, etc., as 

 well as the massive materials used in the manufacture of grind and 

 whet stones. 



A somewhat striking member of this exhibit is the large mass of 

 diatomaceous earth from Lompoc, Santa Barbara County, Calif. 

 The block is some 4£ by 4| by 2| feet in cubical dimensions. The 

 material composing it is about 92 per cent silica, the remainder being 

 mainly water and organic matter, with traces of alumina. The 

 silica portion is composed of approximately 75 per cent of diatoms 

 and 25 per cent of sponge spicules and radiolaria. 



Under ochers or mineral paints are included a number of sub- 

 stances, mainly metallic oxides and carbonates in varying de- 



