24 MR. E. H, CL'NNINGHAM-CRAIG ON [Feb. 1904, 



In the foregoing table (p. 23) these analyses are set forth. It will 

 be seen at a glance that there is a very close resemblance between 

 the albite-gneiss from Cowal (II) and the albite-gneiss from Loch 

 Lomond (III), not only in the actual percentages, but in the ratios 

 of one base to another. I may here remark that the discrepancy 

 between the ratios of potash to soda in the two rocks may be more 

 apparent than real, as Dr. Pollard made three separate estimations 

 of the alkalis in III from different samples of the same specimen, 

 checking his results by an estimation of the silica, and so proving 

 that the analyses were absolutely correct. These analyses gave 

 somewhat different results, as follows : — 



a. h. 



K 2 3-88 3-97 327 



Na o 2-38 2-49 3-15 



Result b was obtained from the largest amount of material, and 

 is accordingly given in the table, but it will be seen that c gives a 

 ratio practically the same as the ratio of potash to soda in II. If 

 the mean of the three analyses be taken, we get 3*70 per cent, 

 of potash to 3 of soda, a ratio not differing very greatly from 

 that in II. 



The next point brought out by these analyses is the great 

 dissimilarity between the phyllite (I) and either of the albite-gneisses 

 (II & III). The low percentage of silica, lime, and soda, and the 

 high percentage of alumina, iron, magnesia, and potash make this 

 clear at once ; while if the ratios, for example, of potash to soda, be 

 taken, the difference becomes even more conspicuous. It is evident 

 that a phyllite of such composition could not possibly form an 

 albite-gneiss. 



When we turn to the analysis of the schistose grit (IV), it does not 

 seem at first to resemble those of the albite-gneisses very closely, 

 but it must be remembered that this rock was selected as being the 

 most siliceous that could be obtained. Consequently, we find a very 

 high percentage of silica, and the other constituents are reduced in 

 proportion. Taking the ratios of one constituent; to another, the 

 resemblance between the grit and the albite-gneiss becomes very 

 striking : thus the ratio of alumina to lime or magnesia in the grit 

 approximates fairly well to the ratio of the same constituents in 

 the albite-gneisses, while the ratio of potash to soda is exactly 

 the same as in the albite-gneiss specimen from Cowal. From 

 these facts it is evident that an albite-gneiss could be formed even 

 from a highly-siliceous grit. 



VII. The Zones of Progressive Metamorphism. 



The sketch-map (fig. 3, p. 25) shows approximately the different 

 zones in the metamorphism of the Beinn-Ledi Group : first that in 



