308 



J. A. THOMSON. 



presence of extremely minute inclusions, while the ortho- 

 clase is practically clear. The latter occurs in but slight 

 amount in the more basic rocks, and is always interstitial 

 to all the other elements, and generally crowded with 

 needles of apatite. In those rocks containing quartz it is 

 more abundant as an element of the micropegmatite. The 

 plagioclase is a thoroughly basic variety witli only at most 

 a slight peripheral zoning of acid material. Its refractive 

 indices are always greater than those of balsam and of 

 quartz whenever a comparison is possible. Carlsbad, albite 

 and pericline twinning are abundant, and the extinctions 

 indicate, by Levy's method, basic labradorite and bytownite. 



One of the rocks, No. 12, was kindly analysed by Mr. 

 G. J. Burrows, in the geological department of the Sydney 

 University. With it may be compared some similar analyses 

 made on the same class of rock elsewhere. 





I. 



II. 



III. 



IV. 



V. 



Si0 2 



51*55 



50*55 



50*76 



52*49 



8592 



Ti0 2 



0*53 



005 



0-46 



0-62 



66 



A1 2 3 



18*85 



17*16 



16*83 



16-44 



1848 



Fe 2 3 



0*32 



1-04 



4*16 



2*60 



20 



FeO 



6'77 



3-40 



4*45 



5*30 



940 



MnO 



0*13 



0*19 



0*69 



trace 



18 



MgO 



7*09 



9-97 



10*09 



6*18 



1772 



CaO 



14*04 



14*77 



11*30 



11*71 



2507 



Na 2 



0-63 



1-62 



0*97 



2*06 



102 



K 2 



0*08 



o-ii 



0*06 



1*09 



9 



H 2 



o-ii 



0*36 



0*14 



1*42 





H 2 



0'04 



0*12 



... 



0*15 





p 2 o 5 



trace 



... 



none 



trace 





00 2 



none 



... 



... 



• •• 





FeS 2 



none 



0-17 



... 



99*91 







100*14 



99-51 



100*06 





Analysis, Burrows. Simpson. Ditrich. Harrison. 



