PETROGRAPHICAL NOTES. 83 



In the hand-specimen the rock has a pale green colour. Though 



it is not very fine-grained, the component minerals are difficult to 



distinguish because they all possess nearly the same colour. A few 



specks of pyrites may be recognised. The specific gravity is 2*91. 



Under the microscope the rock shows an ophitic structure with a 



tendency to glomeroporphyritic. The augites occupy large areas 



without proper crystallographic boundaries and are penetrated 



into by the labradorite prisms. The crystallisation of the felspar 



had gone on to a considerable extent before that of the augite 



began. The mineral belongs to the variety 

 Unaltered augite. , . ,. , . , ,, 



known as sahlite, being almost colourless in 

 section and very transparent in the hand-specimen. The cracks and 

 cleavage striations are more developed than is usually the case in re- 

 cent lavas, but otherwise the mineral is absolutely fresh and un- 

 altered. It seems as though these pale augites are not liable to 

 paramorphic changes in the same measure as the darker, brown 

 varieties. Not only is it the least altered mineral in the rock, 

 but it is the only one unaltered. The felspars are considerably 

 obscured by chlorite, epidote and actinolite which surrounds the 

 crystals, penetrates round their borders or through their cleavage 

 cracks, or form patches in the midst of their mass. The abundance 

 of these secondary minerals attests the former presence of some 

 ferromagnesian minerals other than the large pale augites which 

 evidently contain very little iron. 



As a contrast to this rock with its unaltered augite, I may next 

 mention a specimen containing the most typical uralite ^fe. It was 

 collected north of Sisla Dan and belongs to the more northern of the 

 two bands of volcanic rocks represented on figure 5. The texture 

 exhibits about the same coarseness of grain as in the rock y 1 ^ just 

 described, only the hand-specimen presents a mottled appearance 

 owing to the contrast between the dark green silky hornblende and 

 opaque white saussuritised felspar. Idiomorphic crystals of secondary 

 pyrites, cubic in shape, are abundant. The specific gravity is 2*97. 

 Under the microscope the structure appears well preserved, 

 G2 ( 83 ) 



