PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 763 



Silica-kaolin.] 



No. 1701. SILICA-KAOLIN. 



Same place as No. 1700. 



Ref. Annual Report, xxi, page 155. 



M'f/. Similar to No. 1700, but harder aud finely banded, white and brownish. 

 This specimen is closely similar to No. 70(S), the description of which is as follows:* 



" Specimen No. 70 is a consolidated fine white powder, with a greasy feel. It 

 can be easily cut with a knife into any desired shape. It is mainly white, with 

 frequent bands of brown or light red. That these bands are later in origin than the 

 reduction of the rock to its present condition, and result from the precipitation of 

 iron oxide from infiltrating waters, along the weakest zones, is quite certainly shown 

 by a study of this and other specimens from the same place. For while the bands 

 are in general beautifully distinct and parallel, yet they take advantage of any 

 weaker line which offers, whatever the direction, and offshoots from the main band 

 may be found, striking off at an angle, and generally terminating blindly. This 

 material was at first supposed to be a kaolin, and later, from its evident origin and 

 by analogy with other powders of like nature, but developed on a smaller scale, it 

 was suspected to be a silica powder. An analysis was made of the white portion of 

 the rock with the following result: 



Analysis of No. 70 (Chemical Series No. 238) by C. F. Sidener. 



Silica SiO 2 77.89 per cent 



Alumina A1 2 O 3 13.55 " 



Sesquioxide of iron Fe 2 O 3 1.83 



Lime CaO trace 



Magnesia MgO .36 per cent 



Potash K 2 O .84 



Soda Na 2 O .58 " 



Water H 2 O 4.45 



Total 99.50 



" This analysis shows the rock to be mainly a pure silica powder, with a large 

 amount of the hydrous silicate of alumina, or kaolin; a very small residual portion 

 of the decomposed and leached iron oxides, here evidently in the form of the hydrous 

 sesquioxide; and small amounts of the calcium, magnesium, potassium, and sodium 

 which entered into the composition of the rock from which this was derived." No 

 section. 



Age. Animikie (iron-bearing member) or Cretaceous kaolin. u. s. G. 



No. 1702. SILICA-KAOLIN. 



Same place as No. 1700. 



Ref. Annual Report, xxi, page 155. 



Meg. A banded rock similar to Nos. 1700 and 1701, except that it is hard and 

 flinty. No section. 



Age. Animikie (iron-bearing member), or Cretaceous kaolin. u. s. G. 



* J. E. SPUBR. Bulletin x, pp. 81, 82. 



