374 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



cases having the composition of pure sericite, in others of pure 

 kaolinite, and in others of mixtures of the two. A specimen of clay 

 from the Garfield tunnel at Silver City which contained abundant 

 marcasite in arborescent forms proved upon analysis to correspond 

 in composition to a mixture of kaolinite and sericite. A "talc" 

 clay filling fissures in the Henrietta mine and rich in miargyrite 

 proved to have a similar composition. The Ontario vein on Florida 

 Mountain contains much thin white clayey material. A specimen 

 from the tunnel 610 feet from the portal contained: 



Per cent 



Water (H 2 0) below 100° C 0. 33 



Water (H 2 0) above 100° C 2.75 



Potash (K 2 0) 12.91 



Soda (Na 2 0) . 35 



This, which neither to the naked eye nor by the microscope can be 

 distinguished from kaolinite is clearly a nearly pure muscovite. 

 The Tip Top vein in the drift 70 feet down from the collar of the 

 shaft, consists of 4 feet of light brownish "talcose" clay which 

 contains : 



Per cent 



Water (H 2 0) below 100° C 0. 82 



Water (H 2 0) above 100° C 3. 38 



Potash (K 2 0) 11. 98 



Soda (Na 2 0) . 34 



This also is evidently nearly pure sericite. A 4-inch seam of 

 light-gray clay from the hanging wall of the same vein contained : 



Per cent 



Water (H 2 0) below 100° C 5. 53 



Water (H 2 0) above 100° C 10. 04 



Potash (K 2 0) 3. 21 



Soda (Na 2 0) . 18 



This latter corresponds to a mixture of sericite and kaolin. 



Guided by the above partial analyses, which were made by Hille- 

 brand, Lindgren classified the clays as sericite or as mixtures of 

 sericite and kaolinite on the basis of alkali content. This means 

 does not consider the extent to which the potash, essential to mus- 

 covite proper, may be replaced by water in sericite. By this rule 

 the sericite described as analyzed from Boise County, which contains 

 only 6.38 per cent of potash and 6.06 per cent of water, as compared 

 with the 11.8 per cent of potash required for the muscovite formula, 

 would be classified as such a mixture of muscovite and kaolinite. 

 Its homogeneity, in the analyzed sample, is unquestionable. The 

 same may be true of the clays from Owyhee County, which have 

 been considered as mixtures. One of the supposed mixtures of 

 kaolinite and sericite from the Silver City region has been reexamined 

 by the writer and the description has been published in the paper 

 on gouge clays above cited. This is the clay associated with arbore- 



