160 PETROGRAPHIC PROVINCE OF NEPONSET VALLEY, MASS. 



andesite, 7.4, with an increase in the andesitic dike to 26.1. In the rhyolitic lava 

 the feldspar is purely alkaline and in all the types the lime molecule is very 

 inconsiderable except in the andesitic dike, where it nearly equals the albite 

 molecule producing a labradorite of the approximate composition AbiAn^ 



Corresponding to the low magnesia percentage is a remarkable paucity in the 

 ferromagnesian constituents; because of the alteration which these rocks have 

 undergone, the nature of the melanocratic constituents can only be conjectured 

 from the decomposition products and from the analyses ; their amount does not 

 exceed 11 per cent, in the acid material or 16 per cent, in the basic material. 



Oxide Ratios. 

 Na 2 : K 2 0. In all the rocks of the Neponset igneous complex the percentag 



23 



of soda exceeds that of potassa : in the batholith the alkali ratio is highest in the 

 granite (1.902) and lowest in the rhyolite (1.106); in the aporhyolite intrusives 

 it is highest in the lava (3.563) and lowest in the center of the dikes (2.355); in 

 the trachyte it is very high (17,500), higher than in any other type; in the andesite 

 it is higher in the lava (8,692) than in the intrusive (1.00); in only one case is 

 the ratio a whole number, nor does it often approximate a multiple and one half. 

 In any one group of rocks there is greater range in the soda than in the potassa 

 which remains comparatively stationary; if the trachyte and andesite be ex- 

 cepted, this is true of the whole series. In general Na 2 increases relative to K 2 

 with the decrease in Si0 2 as has been found to be the case in the Essex 21 and 

 Christiania regions ; 22 but this is not a uniform relation ; the departure from this 

 relation is greatest in the trachy te-andesite group where the relation is reversed ; 

 the highest alkali ratio is found in the most acid rock and the lowest in the most 

 basic rock ; this law of variation has also been found to hold true in other regions 



FeO : Fe 2 3 . The variations in this ratio are slight and decrease is in direct 

 proportion to the decrease in Si0 2 . It ranges from 2 in the most acid rock, 

 the fine-grained granite, to 0.576 in the most basic rock, the andesitic dike. 



Na 2 + K 2 : Si0 2 . This ratio, of course, increases somewhat with the de- 

 crease in Si0 2 ; it shows comparatively little variation ranging from .038 to .140, 

 in the aporhyolitic series it is almost constant (.083, .09, .09). 



A1 2 3 + Fe 2 3 : CaO + Na 2 + K 2 0. This ratio, in general, increases with 

 the decrease in silica and is never far from unity; the range is from .89 in the 

 most acid rock— the fine-grained granite — to 1.247 in the andesitic lava; an ex- 

 ception to this statement is the ratio in the case of the border of the aporhyolite 

 dike; the high ratio in this case (1.32) is due to an increase in Fe 2 3 , and to a 

 less degree of A1 2 3 , in the border relative to the center. . 



A1 2 3 : CaO + Na 2 + K 2 0. This ratio is also very uniform and never tar 

 from unity; it cannot be said to vary with the acidity of the rock. 



Co., p. 467, Jour, of Geol, vol. VII, 1899. 



Washington, Petrographical 



x 



p. 203, 1892. 



Brogger, Eruptivegesteine des Kristianiagebietes, vol. Ill, p. 249, note 1, 1897. 

 Pirsson, Bull. 139, U. S. Geol. Survey, p. 138, note 5, 1896. Harker, Geol. Mag 



