VOLCANIC NECK AT THE BASIN, NEPEAN KIVEK. 



139 



Some sections, evidently basal, are completely isotropic 

 and the mineral is uniaxial with negative character. The 

 refractive index is lower than that of Canada balsam. It 

 has not been possible yet to examine the mineral separately 

 and determine <*> and e, but the D.R. measured on a section 

 *039 mm. thick was calculated at '0045. Sometimes, an 

 apparently homogeneous grain, will, under crossed nicols, 

 exhibit a division into an isotropic central portion and a 

 number of surrounding feebly doubly-refracting areas. 1 



If the above characters are themselves not abnormal^ 

 then the data given suggest some such mineral as apo- 

 phyllite or chabazite. The occurrence, however, is not 

 that of crystals lining cavities or cracks as one might 

 expect in the case of such minerals. 



The mineral in question alters to irregular masses which 

 grade almost imperceptibly into the fresh mineral, these 

 resultant masses showing high birefringence. The effect 

 of pressure on this assemblage of minerals has been to 

 produce a large number of parallel Assures into which 

 secondary solutions have found their way, and now the 

 quartz shows a perfect network filled with a strongly 

 birefringent material which has also a high index of refrac- 

 tion. Many of the quartz fragments have been detached 

 and lie embedded in the basalt* Whether selective assimil- 

 ation was the only factor in effecting this separation is 

 hard to say. There is often developed between the separ- 

 ated crystals of quartz and the compact rock inclusion a 

 vein of bovvlingite with its distinctive characteristics. 



The third section is made up almost entirely of the 

 unknown mineral, which is idiomorphic towards some 

 irregular, more or less rounded areas, now much confused 

 and stained, but suggesting broken down pseudomorphs 



1 Cj., Iddings, «■ Rock Minerals/' p. 287. 



