WARREN. — ALKALI-GRANITES AND PORPHYRIES. 283 



of alumina will then fall to the pyroxene and hornblende to agree with 

 the general alumina-poor character of these minerals in the Quincy 

 magma; also considerable free quartz would result, whereas little is 

 really present. It, therefore, appears that albite is considerably 

 more abundant, in the groundmass, than in the rhombenporphyry 

 (and this agrees with microscopic observations so far as it is possible 

 to judge in so fine-grained a rock). Working on the assumption, that 

 with the exception of some soda combined with the ferric iron, this 

 oxide goes into albite, we may venture a very rough approximation 

 of the mineral composition as follows : — 



Feldspars 65 . 



Pyroxene, Hornblendes, etc. 30.0 



Accessories 5 . 



100.00 

 Although less feldspathic and richer in pyroxene, etc. than the average 

 of the rhombenporphyry, the xenolith is more siliceous owing to the 

 greater amount of silica called for by the albite and by the soda- 

 pyroxenes and hornblendes. 



It is easy to trace with the microscope the gradual change from the 

 moderately porphyritic type of rhombenporphyry, which is developed 

 in the larger masses near the slate, with only the colorless or pale 

 brown augite, through types, occurring as smaller masses down to 

 those comparable in size with the average xenolith, in which pyroxene 

 of the green, sodic type becomes more and more abundant, in which 

 sodic-hornblende becomes also gradually more abundant and in which 

 quartz may appear sporadically, to a type like the one analyzed. 

 From this we pass with increase of silica, decrease of iron and lime, 

 into types which resemble closely some phases of the granite por- 

 phyry of the lower and thinner contact zones. 



Turning now to the quartzose and aegirite rich xenoliths we may 

 note that in contrast with its enclosing granite (No. 3) that it is lower 

 in siHca, and that the iron is lower and nearly all ferric, as would be 

 expected from the presence of aegirite alone without the ferrous-iron 

 bearing hornl:)lendes found in the granite. 



The more siliceous, finely granitic and slightly or non-porphyritic 

 xenoliths are often almost the exact counterparts of some of the fine- 

 granite of the contact zones, others seem to depart somewhat from this 

 type. But as has been pointed out, the fine-granite from different 

 localities (diiferent parts of the contact zone) varies somewhat in com- 

 position, and it is probably true that there is a close resemblance 

 between the members of these types and the various fine-granites of 



