380 Transactions. — Geology. 



for these combinations will be present as tridymite, as no 

 interstitial glass has been detected. 



Taking the average composition of sanidine as that given 

 in Dana's " Text-book of Mineralogy," we find that it contains 

 16-9 per cent. K2O and 64-7 per cent. SiOo. In a rock con- 

 taining 2-46 per cent. K.^O, 9-42 per cent. SiOo would be 

 required for combination. Again, CaO and Na.O make up 

 14-7 per cent, of andesine, while the percentage of silica is 

 59-8. Assuming that NaaO and CaO are to a limited extent 

 mutually replaceable in the andesine molecule, and taking the 

 molecular weight of Na.2 (46) as equivalent to that of CaO (40), 

 we have in this rock 8-06 per cent, of these bases, and they 

 would require 32-60 per cent, of silica for complete saturation. 

 Altogether, then, 42-02 per cent, of silica is required for com- 

 bination, and the remaining 29 per cent, will be represented 

 by tridymite. As, however, a very small proportion of the 

 rock as seen in section is tridymite, it is fair to assume that 

 a considerable amount must exist in the ground-mass. 



As tridymite is stated to be soluble in caustic soda 

 (Eosenbusch : " Microscopical Physiography of Eock-forming 

 Minerals," translated by Iddings, p. 174), an attempt was 

 made to estimate its percentage by taking advantage of that 

 fact -. 29-95 per cent. SiOa was dissolved out of the rock, but, 

 as 15-05 per cent. AI2O3 was also present in the solution, the 

 result is not satisfactory. 



An analysis was also made of the feldspar, but it was found 

 extremely difficult to isolate an appreciable quantity, and the 

 result of the analysis cannot be considered strictly accurate : — 



SiOo ... ... ... ... 60-56 



AI063 ... ... ... ... 22-05 



CaO ... ... ... ... 7-25 



K2O ... ... ... ... 3-54 



Na.O ... ... ... ... 7-93 



101-33 

 The result is almost the same as would be expected on con- 

 sideration of the bulk analysis of the rock, except that the per- 

 centage of K2O is rather high. Sanidine forms only a small 

 border round the relatively large crystals of plagioclase that 

 were the only ones obtainable for analysis. 



No attempt was made to determine the species of plagio- 

 clase by means of its specific gravity, as the difference in this 

 physical quantity between two closely-allied species of plagio- 

 clase is extremely small, and would be completely masked by 

 the lower specific gravity of the mantle of sanidine. An ap- 

 proximate correction for this would give an eminently unsatis- 

 factory and wholly unreliable result. The same applies to 

 Szabo's flame reactions. 



