PETROGRAPHICAL NOTES. 85 



points of difference are its texture which is finer-grained, and its 

 structure which is more ophitic. The degree of alteration of the 

 minerals is perhaps less, but the mode of alteration is exactly the 

 same. The original structure is very well preserved. Even the 

 original twinning of the augites is reproduced in the uralite. 



Such is again y^y which was found about half way between Girwi 

 and Naogai, but not in situ; it was a spheroidal boulder. It is greyish 

 green in colour, with a mottled appearance in the hand-specimen. 

 The specific gravity is 2*91. It contains the same minerals as yy 1 ^ 

 with the addition of a large proportion of calcite. The original 

 structure is difficult to make out exactly owing to the changes that 

 have taken place : it seems that the original augites had crystallised 

 largely before the felspars. They are now entirely changed to uralite, 

 and in a few cases the frayed edges of the crystals show a slight 

 bending which indicate that the structure may be partly due to 

 subsequent movement in the rock mass. At the same time we have 

 undoubted proof of the secondary growth of the hornblende. Where 

 the uralite comes in contact with some of the large patches of the 

 easily yielding mineral chlorite, it has grown into those spaces, 

 assuming the shape and the characteristic sharp angles of the amphibole 

 prism. The original limits of the crystal may sometimes be recognised 

 owing to their green colour, the additional growth consisting of 

 colourless actinolite. 



In the midst of the chlorite patches there occur also pale hornblende 

 crystals which are perfectly idiomorphic. It is probable that these 

 do not represent any paramorphic change of augite originally situ- 

 ated at the place which these crystals occupy, but they are entirely 

 new formations derived from material furnished possibly by minerals 

 of various species. Calcite and chlorite form large patches through 

 the uralites. The calcite often forms idiomorphic crystals through 

 the chlorite aggregates. The felspars are somewhat clouded owing 

 to kaolinisation, the effect of atmospheric weathering. They are 

 otherwise almost unaltered. Leucoxene occurs in the same manner 

 as in the rocks already described, sometimes with a small unaltered 

 nucleus of ilmenite. 



( 85 ) 



