WARREN. — ALKALI-GRANITES AND PORPHYRIES. 297 



However, had it been assumed that the albite was as sodic as Ab98An2 

 instead of AbgsAns, the ratio would, for the average of the granites (No. 

 4), been only 1.21 at most, and this would have reduced the aegirite 

 and hornblende percentages considerably below their true value and 

 raised the magnetite above a probable value. A similar change in the 

 albite, would in the case of the fine-granite, affect the ratio very 

 little since this was controlled by a cjuantitative, microscopic estimate 

 of the minerals present, and this ratio also falls just below Vogt's 

 minimum. The Quincy rhombenporphyry, however, does fall within 

 it. Despite the possible errors (at least these are perhaps no greater 

 than enter into Vogt's admittedly approximate estimates) in such 

 calculations where some assumptions have to be made, and where 

 slight changes in the Xa2() values used cause much greater changes 

 in the resulting minerals, the writer believes that the values given 

 are worthy of some weight and indicate a wider fluctuation in the 

 proportions of soda to potash feldspar in their intergrowths than 

 those estimated by Vogt. It is also to be noted that a small portion 

 of the albite occurs in these rocks outside of the intergrowth. This 

 is included in the above calculations because of the impossibility of 

 estimating its amount, but if allowances for it could be made it 

 would have the effect of still further lowering the proportion of albite 

 that is present in the intergrowth with the microcline. 



The relative proportions of quartz to feldspar are also interesting. 

 Table II, p. 296, shows that these are, when calculated to 100%: — 

 37.4%, quartz to 62.6%;, feldspar in the coarse-granite; 25.9% to 

 74.9% in the fine-granite and 29.5% to 70.5% in the granite-porpliyry. 

 Although there is admittedly some chance of error in the calculation of 

 these percentages, the writer believes that the possible error is not 

 sufficient to account for the considerable differences shown ])y these 

 figures and that they represent real differences of composition. While 

 the fine-granite and granite-porphyry are not so far apart, the coarse- 

 granite shows a wide divergence. Dispite the probability that the 

 considerable amounts of sodic-iron silicates present in the rocks might 

 effect the composition of a possible ciuartz-feldspar eutectic as com- 

 pared with purer quartz-feldspar granites by an amount impossil)le to 

 estimate at present, it is interesting to note that the figures for the 

 fine-granite fall on one side of the ratio for the "granite eutectic" as 

 estimated by Vogt ^° and the granite-porphyry on the other side. 

 Vogt's estimate is quartz, 27.5% feldspar, 72.5%. The coarse-granite 



50 Tscherm. Min. Pet. Mitth. (2), 25, pp. 361-2, 383-5 (1906). 



