l88 HOLLAND : SIVAMALAI SERIES. 



This analysis shows a mixture of potash, soda and lime felspars 

 in about the ratio of 5 Orthoclase : 8 Albite : 2 Anorthite, a mixture 

 which would give, theoretically, the following analysis . — 



Silica ....... 65*22 



Alumina 

 Lime 

 Potash 

 Soda 



20*50 

 1-48 

 623 



6'57 



IOO'OO 



, The chemical composition of the felspar agrees with the variety 

 so common in the elaeolite-syenite family which Rosenbusch has 

 proposed to call anorthoclase. But the microscopic sections give 

 the characters of microperthite ; the principal mass of the mineral 

 being monoclinic in its optical behaviour, whilst the long spindle- 

 shaped inclusions show hignly inclined extinctions on the clino- 

 pinacoidal sections, with narrow extinction angles on basal sections. 

 The soda and lime are, however, higher than would be expected, 

 unless the soda is accounted for on the supposition that the mono- 

 clinic felspar is the soda-orthoclase which Brog^er suggests is merely 

 a sub-microscopic intergrowth of orthoclase and albite (cryptoper- 

 thite). 1 The microscope shows that the grey felspar is almost en- 

 tirely microperthitic in its structure. In one of the orthopinacoidal 

 sections of this felspar there were found a series of minute opaque 

 inclusions of a black mineral forming ragged laths, four or five times 

 as long as broad, and apparently formed by an aggregation of octa- 

 hedral crystals in parallel grouping, with the long axis of the lath or 

 group of crystals lying in the clinodome. Each black inclusion is 

 surrounded by a zone showing slightly higher double refraction than 



1 Dr. Walker thinks it unwise to base too severe a criticism of the felspar on this 

 analysis, as the chemicals used were by no means above reproach. The 

 microscopic evidence may, therefore, be considered more reliable till a fresh 

 analysis can be made. 



( 20 ) 



