COMPOSITION OF THE SYENITE. 



249 



columnar habit. In natural light the substance of the feldspar appears 

 to be homogeneous, but under crossed nicols it is seen to be an inter- 

 growth of two very different substances with extinctions corresponding 

 .to orthoclase and albite. The orthoclase has suffered the effects of altera- 

 tion to a much greater extent than has the albite, and inconsequence 

 has often entirely disappeared, while its place is now occupied by a 

 cli >udy aggri sgate of kaolin or of micaceous minerals. The albite remains 

 quite fresh, and so includes these secondary products. The other inclu- 

 sions of the albite. as well as those of the orthoclase, are the eleolite and 

 sodalite grains already referred to, with crystals of sphene, apatite, and 

 ( lark green hornblende, and an occasional rounded grain of zircon ; besides, 

 of course, the usual liquid inclusions. It is not certainly known whether 

 other feldspars than those mentioned are present or not, but it is assured 

 by the analysis of the rock that if they do occur it is in but very small 

 quantity. A separation of the feldspar from the powdered rock by a heavy 

 solution points to the same conclusion ; for while a great lot of material 

 fell when the density of the solution was between 2.571 and 2.586, but a 

 trifling, quantity was precipitated on either side of these limits. AA 

 analysis of that portion of the powder whose specific gravity was 2. 57-2. 5s 

 showed it to consist partly of eleolite and partly of feldspar. These were 

 separated by extraction with hydrochloric acid and digestion with sodium 

 carbonate, and then analyzed by Mr. W. F. Hillebrand, who reports these 

 figures : 



Si( )., . 



Fe 2 3 



( !a< ) . 

 BaO. 

 MgO 



K.,( ) . 



X;i..< ) 



Nephelin,e. 



45.3] 

 32.67 



2.00 



Total 



.1(3 

 5.70 

 L2.60 

 L.56(calc. 



Feldspars. 



100.00 



66.85 



19.50 



.13 



.11 

 .07 

 tr. 

 5.80 

 7.11 

 .3] 



100.21 



From the rt 'sul t of this analysis il is quite plain thai the insoluble por 

 tion of the powder is a mixture of orthoclase and albite molecules ; and 

 since the microscope shows the presence of two feldspars in each grain, 

 it is equally certain that these molecules are in the form of intergrowths 

 of orthoclase and albite and not in their combination anorthoclase. 



