290 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The upovhyolite (IS) is more acid than the granite or its contact 

 facies, the total iron lower and the potash relatively higher. The 

 averages given l)y Crosby (No. 19) of the rock from Pine Hill, show- 

 lower silica and higher alumina. Variation is probably to be expected 

 in different parts of a rock of this character. Devitrification and the 

 attendant alteration have undoubtedly modified the rock as a whole 

 and the true composition of the rock as originally solidified cannot 

 now be determined. But there is no doubt whatever of its belonging 

 to the Quincy type of magma. 



The norm is as follows : — 



Quartz 36.84 1 Sai ^ ^.q. Class I. Persalane. 



Orthoclase 27.80 95^-3^,5^^ 



Albite 30.39 [ ^ 



Anorthite .56 J -^ = . 6. Order 3. Quarfelic. 



F 



Corundum .92 1 K20+Na-.o ., j^ , r> n i- 



TT 1 r.^ \ — r^art — = '^4. Kange 1. reralkalic. 



Hypersthene 86 k ^. -g^^ . ^ 



T\/r +v o 00 r 4 . 56 Jb emics. 



Magnetite 2.32 j k,o ^ , ,. 



Ilmenite .46 J nS^ = -86. Subrange 3. bodipotassic. 



The rock is, therefore, an alaskose and may l^e termed a grani- 

 alaskose. The mode is essentially like the norm. The ratio of Ab: 

 Or is 1 .09 that of the granite 1 .02; or Ab, 52.3; Or, 47.7, the gran- 

 ite Ab, 50.5; Or, 49.5. 



In columns 11 and 19 are given the compositions of the rhyolitic 

 rocks associated with the neighboring Xeponset \'alley granite in- 

 trusion. They show the characteristic differences of the two magmas, 

 viz. higher iron and predominance of potash over soda in the Quincy 

 magma, whereas soda dominates, sometimes greatly, over potash in 

 the Neponset rocks. Thus the chemical evidence agrees entirely 

 with the microscopic, in showing that the Blue Hill aporhyolite belongs 

 to the Quincy granite magma and is quite distinct from the abundant 

 ^'olcanics developed in the neighboring areas of granitic rocks. 



Slate-Graxite Contacts, North Common Hill, Quincy. 



The contacts of the coarse-granite with the slate along the northern 

 side of North Common Hill, Quincy, show no intermediate contact 

 phases and therefore call for special mention. As pointed out by 

 Crosby ** there are numerous patches of slate lying in the coarse granite. 



44 loc. cit., p. 28 et seq., also special map opp. p. 428. 



