318 McMAHON: PETROLOGICAL NOTES ON SOME PER1DOTITES, ETC., 



per cent, was dissolved (Mr. Blyth). After ignition gelatinizes with 

 hydrochloric acid. 



Contains abundant silica, alumina and lime, a very little iron, 

 magnesia, soda, and a little water. 



In all the above characters the mineral agrees with zoisite, except 

 that it is decomposed (without ignition) by hydrochloric acid. A 

 specimen of zoisite from Valtigl, Tyrol (M. 1639) when heated in 

 hydrochloric acid for 2\ hours had 10*41 per cent, dissolved. After 

 heating for 2\ hours in strong sulphuric acid 8*4 per cent, only was 

 dissolved of I-177," [viz., the white mineral in 8-294 under consider- 

 ation. New numbers seem to have been given to the specimens 

 after their transfer to the Geological Museum.] 



The specific gravity 3*442 and hardness of 7, alluded to above, 

 appears to refer to the white mineral (zoisite) and not to the rock 

 specimen as a whole. I found the Sp. G. of the serpentine to 

 be 2*67, and the zoisite 3*483, which latter corresponds very 

 closely to Mr. Blyth's figures. I operated on a small piece with the 

 aid of a chemical balance. Dana gives the Sp. G. for " ordinary'* 

 zoisite as 3*226 to 3*381 (Text-book of Mineralogy by E. S. Dana, 

 1898, p. 438). The slight increase in the Sp. G. of the Za*nskar 

 mineral is doubtless due to the presence of the oxide of iron, 

 numerous dots of which are to be seen in thin slices under the 

 microscope. 



Some would probably call the mineral under consideration 

 saussurite and the term would not be inappropriate. Dana puts the 

 hardness of saussurite as 6*5 to 7 and the Sp. G. as ranging from 3 

 to 3*4 (System, 6th Ed., 1892, p. 515). a In composition it often 

 approaches zoisite, of which it has been regarded as a soda-bearing 

 variety." 



As the name saussurite appears to be given to minerals which 



differ much from each other and as " it is rarely, if ever, " Dana 



states, <( a homogeneous mineral," I think it will be best to call the 



white substance in the vein running through the Za*nskar serpentine 



( 16 ) 



