330 



REPOET OF NATIONAL MUSEUM, 



Plate 15, figs. 1 and 2, will serve to show the shape and kind of the 

 particles in the mineral kaolinite and in a prepared sample of the 

 Hockessin kaolin, as seen under the microscope. 



The name halloysite is given to a white or yellowish material closely 

 simulating kaolin in composition, but occurring in indurated masses, 

 with a greasy feel and luster, and which adheres strongly to the 

 tongue, a property due to its capacity for absorbing moisture. 1 As it 

 is utilized for much the same purpose as is kaolin, it is included here. 



Halloysite is described by Gibson 2 as occurring in a bed some 3 feet 

 in thickness, lying near the base of the Lower Siliceous (L. Carbon- 

 iferous) formation, a little above or close to the Black Shale (Devonian), 

 in Murphrees Valley, Alabama. This bed has been worked with satis- 

 factory results near Valley Head, in Dekalb County. The present 

 writer has found the material in comparatively small quantities, asso- 

 ciated with kaolin, in narrow veins in the decomposing gneissic rock 

 near Stone Mountain, Georgia. A similar occurrence is described 

 near Elgin, Scotland. (Analysis below.) Near Tiiffer, Styria, halloy- 

 site is described 3 as occurring in extensive thick and veinlike agglom- 

 erations in porphyry. It is quite pure, and in the form of -irregular 

 nodules of various sizes, frequently with a pellucid, steatitelike cen- 

 tral nucleus, passing outwardly into a pure white substance, greasy to 

 the touch, m which are occasionally included minute pellucid granules. 

 Outside it passes into an earthy, friable substance. The following 

 analyses show the varying composition of halloysite from (I) Elgin, 

 Scotland, (II) Steinbruck, Styria, and (III) Detroit Mine, Mono Lake, 

 California. 



A white chalky halloysite from the pits of the Frio Kaolin Mining 

 Company in Edwards County, Texas (Specimen No. 53253, U.S.N.M.), 



1 This property is characteristic of nearly all clay compounds when they are dry. 

 It is to this same property that many of the so-called "madstone" owe their imagi- 

 nary virtues. Nearly all the stones of this type examined by the writer have proved 

 to be of indurated clay, halloysite, or a closely related compound. When applied 

 to a fresh wound, such adhere until they become saturated with moisture, when they 

 faU away. Their curative powers are of course wholly imaginary. 



2 Geological Survey of Alabama. Report on Murphrees Valley, 1893, p. 121 

 "Mineralogical Magazine, II, 1878, p. 264. 



