296 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



A study of the ratios existing between the sum of the molecular pro- 

 portions of alumina and ferric oxide, to that of the soda and potash 

 reveals a high degree constancy if we except the rhombenporphyry 

 phases (15 & 16 Table No. I). Inasmuch as no allowance has been 

 made for magnetite nor the small amounts of lime and ferrous-iron 

 which, of course, effect this ratio, some variation is to be expected. 

 The departure from constancy in the rhombenporphyries is caused by 

 the larger amounts of lime and ferrous iron, whose relations cannot be 

 very precisely determined, but the direction of the divergence is the 

 one to be expected. 



Another point of considerable interest is the proportions of the soda 

 and potash feldspar molecules present in the microperthite resp. 

 cryptoperthite or soda-orthoclase. These proportions are shown in 

 Table No. II as calculated from the analyses: — 



TABLE NO. II. 



There is, in spite of a rather large fluctuation in the total amounts of 

 the two feldspar molecules, a rough approximation toward a constancy 

 in their relative proportions, but the ratio is lower than the lower limit 

 for feldspar intergrowths as estimated by Vogt. In calculating the 

 Ab: Mic. ratio several assumptions had to be made which would 

 ob\iously affect the ratio so that they are at best only approximations. 



49 Tschermak's Mineral, u. Petrog. Mitt. Vol. XXIV, No. 6, 1906, p. 



