A. K. Coomdrcmodmy — Contributions to Ceylon Geology. 349 



a little over 90° resemble twins, but the basal planes {cc) are not quite 

 in the zone cr, so that this resemblance appears to be deceptive. I am 

 indebted to Mr. L, J. Spencer, M.A., for these crystallographic details. 



The soft, yellow, micaceous band consists of biotite, plagioclase 

 (quite fresh and glassy), greenish - yellow, soft, serpentine - like 

 decomposition-products after pyroxene (?), and minute quantities of 

 garnet and iron-ore. An analysis (No. I) by Mr. W. C. Hancock, 

 B.A., shows that this yellow micaceous band contains a relatively 

 slightly larger proportion of alumina than the corundiferous rock 

 itself. 



The corundiferous felspathic rock consists mainly of orthoclase- 

 microperthite, with also plagioclase, biotite, corundum, and small 

 quantities of garnet, green spinel, and zircon. It is not possible to 

 make quite certain of the total absence of quartz ; a consideration of 

 the amount of alkali which is, according to the analysis, available 

 would indicate the presence of a small percentage of free quartz ; 

 I have not, however, been able to detect any. 



The microperthite is in a very fresh condition, the plagioclase 

 still more so. The corundum in the rock has usually a ' court ' of 

 felspar free from biotite, separating it from the remainder of the 

 rock, consisting of felspar with scattered biotite. 



Professor Sollas, F.E.S., has very kindly made a mineral analysis 

 of the crumbled rock, with the following results : — 



Heavy minerals over 3"34 (chiefly corundum) 7*6 



Orthoclase, s.gr. 2-56 ... 64*2 



Oligoclase (with possibly a little quartz), s.gr. 2-65 ... 23-5 

 Biotite, s.gr. 2-8-2-92 4-7 



100-0 

 Mr. Hancock has chemically analysed the same material (No. II) ; 

 No. Ill shows the same with water removed and ratios calculated to 

 100 ; No. IV, the molecular ratios. 



SiOo ... 



AI263 ... 



FeoOg ... 



FeO ... 



MgO ... 



CaO ... 



jSra,0 ... 



k/o ... 



h;o ... 



101-17 100-37 100-00 



Calculations based on the analysis show that there is a small 

 excess of alumina (above that i-equired for the felspars and biotite) 

 which might be expected to have crystallized as corundum ; this 

 excess, however, is smaller than the mineral analysis would lead 

 one to expect.^ 



1 The presence of free quartz would raise the amount of available excess alumina, 

 as indicated by the chemical analysis. After careful microscopic examination, 

 however, I feel that there can hardly be any appreciable quantity of free quartz 

 present, if any. 



