THE NONMETALLIC MINERALS. 331 



has the composition given below as shown by analyses made in the 

 laboratory of the department: 



Silica ._ 45.82 



Alumina 39. 77 



Potash 30 



Ignition 13.38 



99.27 



The material is somewhat variable, corresponding in part to the 

 halloysite described by Dana, and being nonplastic, and in part being 

 plastic to an extraordinary degree. The plastic portions are almost 

 as gritless as starch paste. Its appearance under the microscope is 

 shown in Plate 16, fig. 1, the interspaces of the visible angular par- 

 ticles being occupied by the past} 7 , almost amorphous material. The 

 particles themselves act very faintly on polarized light, and it is not 

 possible to determine their mineralogical nature. 



The name Indianaite has been given by Cox to a variety of halloy- 

 site found in Lawrence County, Indiana, and which he regarded as 

 resulting from the decomposition of Archimedes (Lower Carbonifer- 

 ous) limestone. It is represented as forming a stratum from 6 to 10 

 feet thick, underlying a massive bed of Coal Measure conglomerate 

 100 feet thick and overlying a bed of limonite 2 to 5 feet thick. The 

 material like kaolin is used in the manufacture of porcelain ware. 

 The composition of this material as given by Dana is as follows: Sil- 

 ica 39 per cent, alumina 36 per cent, water 23.50 per cent, lime and 

 magnesia 0.63 per cent, alkalies 0.54 per cent; 99.67 per cent. (See 

 Specimens, Nos. 29714, 34441, U.S.N.M.) 



The potters' and pipe clays belong mainly to what are known 

 geologically as bedded clays, and are as a rule very siliceous com- 

 pounds, carrying in some instances as much as 50 per cent of free 

 quartz and 6 to 10 per cent of iron oxides and other impurities. 

 They are highly plastic and of a white to blue, gray, or brown color 

 (See Specimens, Nos. 17245, 33975, 20286, 67796, to 67798, from the 

 United States and England) and burn gray, brown, or red. The tables 

 on page 349 will show the varying composition of materials thus 

 classed. The fire clays, so called on account of the refractory nature, 

 differ mainly in the small percentages of lime and the alkalies they 

 carry, and to the absence of which they owe their refractory proper- 

 ties. (Specimens, Nos. 11629, New Jersey; 53179, Maryland; 59258, 

 West Virginia; 68248, California; 53249-53251, South Dakoka, etc., 

 are characteristic.) 



The bedded clays of the United States reach their maximum devel- 

 opment in strata of Cretaceous and Carboniferous ages. To the Cre- 

 taceous age belong the celebrated plastic clays of New Jersey and a 

 very large proportion of the brick, tile, and terra cotta clays of Dela- 



