532 REPORT OF NATIONAL MUSEUM, 1890. 



lime, and magnesia with more or less of the alkalies, potash, and soda, 

 and iron oxides. In the process of decomposition these soluble por- 

 tions are leached out leaving the less soluble silicate, or kaolin, behind 

 in a condition of more or less purity. The mineral is of great value 

 for fictile purposes, and a larger number of localities are represented 

 in the economic series in the southwest court. Other and more im- 

 pure varieties in the form of clay are to be found with the fragmental 

 rocks. But a few samples from North and South Carolina (39028 

 and 70172) and the Yellowstone National Park are here shown. That 

 from the Yellowstone is thrown up in the form of a thick mud by 

 hot springs (specimen 12879). 



(4) Sulphur, sulphides, a.nd sulphates. — The mineral sulphur 

 sometimes occurs in nature in such masses as to be fairly entitled to 

 consideration as a rock. Its mode of occurrence has been already de- 

 scribed under the head of chemical elements constituting rocks. In 

 combination with iron, copper, lead, zinc, and other metals formiug 

 sulphides, it is an important rock constituent, and often occurs in beds 

 or veins of such dimensions as to constitute a valuable ore. In combi- 

 nation with oxygen and the metals it forms sulphates. Gypsum, the 

 hydrous sulphate of calcium, is a soft, white, yellowish, or pink rock, re- 

 sulting mainly as a chemical deposit from the evaporation of sea water, 

 although, as stated by Geikie (see p. 121), it may originate through the 

 decomposition of sulphides and the action of the resultant sulphuric 

 acid upon limestone; through the mutual decomposition of the carbon- 

 ate of lime and sulphates of iron, copper, etc.; through the hydration 

 of anhydrate, or through the action of sulphurous vapors and solutions 

 from volcanoes acting upon the rocks with which they come in contact. 

 It occurs in beds belonging to various geological horizons, and is usually 

 associated with clay, rock salt, and anhydrate. 



Alabaster is a fine white variety of gypsum used in smaller works of 

 art. (Specimens 36S72 and 38817.) 



Anhydrite is an anhydrous variety of calcium sulphate, somewhat 

 less common than gypsum. Barite or "Heavy Spar," the sulphate of 

 barium, also occurs in nature, but less abundantly than the calcium 

 sulphates. 



The following localities and varieties are represented: 



Sulphur : 



Tehama County, California, 30118; Rahbit Hole Sulphur Mines, Humboldt 

 County, Nevada, 35511 and 35512; Cove Creek, Utah, 35513 (7 specimens) ; 

 Hell Roaring Mountain, Yellowstone National Park (72877); Murcia Prov- 

 ince, Spain, 4246 ; Volcano of Popocatepetl, Mexico, 64773. 

 Sulphides: 



Sulphide of iron. Deer Isle, Maine, 36021. 



Sulphide of zinc. Deer Isle, Maine, 36016; Madison County, New Hampshire, 

 70094 ; Joplin, Missouri, 65262. 



Sulphides of copper and iron. Gilpin County, Colorado, 15083 ; Ascension Island, 

 72950. 



Sulphide of lead, Missouri, 39072; Brittany 6825. 



