56 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 6G. 



The chlorite thus approximates with moderate closeness to the 

 formula, 



5FeO. 5MgO. 2AI3O3. 5SiO,. GH.O, 



or, H,,Fe,Mg,Al,Si.,033. 



It is assumed that the iron is all ferrous although the material was 

 too scanty to permit determination of its state of oxidation. This 

 composition is intermediate and is not definitely referable to any of 

 the named chlorites. It is doubtless related to diabantite and deles- 

 site but, for the present, no specific name will be applied to it. The 

 material, as found, seems to be purely an introduction by solutions 

 and not an alteration product of any constituent of the immediately 

 adjacent rock. Paragenetically it is probably one of the earliest 

 Introductions into the vein, being essentially contemporaneous with 

 the epidote and slightly earlier than the hornblende. 



ASBESTIFORM HORNBLENDE. 



In many specimens, especially of the lot collected from the shear 

 zone in the north central part of the quarry in October, 1922, there 

 is a snow white fibrous amphibole very similar to or identical with 

 that described as occurring in the miarolitic cavities. This forms 

 thin sheets of very fine fibers having a silky luster which occur coat- 

 ing joints in bleached and altered diabase along an east-west shear. 

 These sheets are of paperlike consistency and can be peeled from 

 the rock, giving a typical " mountain leather." The fibers are 

 apparently oriented and the sheet, when placed flat, gives, from the 

 aggregate of very fine fibers, a biaxial interference figure, with an 

 acute negative bisectrix nearly perpendicular to the sheet and 2V 

 large. The extinction in these fibers is strictly parallel. The min- 

 eral is insoluble in boiling concentrated hydrochloric acid although 

 enough iron is extracted to color the solution yellow. A very small 

 portion, only 14 milligrams, of this ''mountain leather" was used for 

 an analysis which obviously could yield only approximate results 

 on so little material. This gave the following percentages: 



Analysis of " mo^mtain leather." 



SiOa 40.14 



AI2O3 6.34 



FeO 27.24 



CaO . 4.93 



MgO 13.38 



Total 92.03 



The analysis serves to confirm the optical identification of the 

 mineral as hornblende and to show that, despite its very white color, 

 it is high in iron. 



