WARREN. — ALKALI-GRANITES AND PORPHYRIES. 281 



smooth glaciated ledges which form Kitchimakin Hill. They vary 

 in size from small fragments an inch or two across to those measuring 

 upwards of a foot on a side. In color, when not oxidized, they are of 

 a light to medium gray or bliush-gra\'. They are feebly porphyritic 

 containing a few small phenocrysts of feldspar (rarely cpartz) up to 

 two millimeters in length. Microscopically they are found to consist 

 of the same feldspar as the enclosing porphyry, quartz and riebeckite, 

 magnetite and alteration products. The riebeckite is partly in the 

 form of abundant small prismatic or flaky aggregates enclosing the 

 grovmdmass minerals and in part in the form of tiny shreds and fibers. 

 The quartz and feldspar are xenomorphic and somewhat variable in 

 grain. The feldspar phenocrysts are often broken, as are the few 

 quartz phenocrysts that have been noted in these inclusions. These 

 inclusions resemble most nearly some of the fine-grained contact 

 phases of the porphyry and doubtless are derived from such a source, 

 having been broken off and included in the still fluid mass beneath 

 the contacts, and then sunk, or were carried away, from their original 

 place of formation. 



Chemical characters. — Analyses of a number of the various types 

 of xenoliths described would doubtless yield interesting results. The 

 writer has, however, confined himself, to two types, partly on account 

 of the labor involved and partly because it is believed that the micro- 

 scopic evidence is sufficient to show the connection chemically and 

 texturally between the various types. The two types chosen are 

 extreme ones. One is a rhombenporphyry type resembling mega- 

 scopically very closely the moderately porphyritic type of the rhomben- 

 porphyry, and came from a recently quarried block of granite from the 

 northern part of the Pine Hill Tract not far south of the type locality 

 for the rhombenporphyry. The microscope showed that the feldspar 

 was the same as in the type rhombenporphyry; cpartz is lacking 

 almost entirely; augite is present but is much less abundant than a 

 green, feebly pleochroic soda-iron type; soda hornblende of tlie green 

 and blue types is present, and some black oxide minerals; some 

 secondary biotite and other alteration materials are also present. 

 The second chosen was a fine granite type of xenolith, the most quart- 

 zose and aegiritic found, and was taken from the granite of the Hard- 

 wick Quarry, Quincy. The results of duplicate analyses follow on p. 

 282. 



In comparison with the chemical composition of the type rhomben- 

 porphyry the xenolith is slightly higher in silica, also in total iron 

 oxides, and the ferric iron is proportionally higher as might be expected 



