274 PROCEEDINGS OF THE AMERICAN ACADEMY. 



granite, and doubtless underlies it e\'erywhere by only a few feet or 

 yards, the xenoliths are relatively abundant, so much so in fact, that 

 all attempts to use the stone commercially have been unsuccessful. 



At a point a few hundred feet west of Willard Street, northwest of 

 Pine Hill (see special map), several good sized masses of the rhoml)en- 

 porphyry occur included in the granite, and with them are smaller 

 masses grading down to those only a few inches across. These mega- 

 scopically and microscopically differ in no essential particular from the 

 less highly porphyritic phase of the type rhombenporphyry, previously 

 described, except that they show a small development of an alkali- 

 hornblende poikilitically enclosing the groundmass feldspar, and in 

 this respect show a gradation toward the granite-porphyr^^ Again on 

 the eastern side of the area, beside the railroad track, is a beautifully 

 glaciated ledge where is to be seen the finest exposure of xenoliths 

 anywhere in the entire field. The granite is here of the porphy- 

 ritic type and is literally packed with xenoliths, chiefly of the dark, 

 fine grained porphyritic type, and it is here that they can be studied 

 to the best advantage. In size they vary from tiny patches consisting 

 sometimes of a single feldspar phenocryst, surrounded by a few milli- 

 meters of dark groundmass, to masses two to four feet in length by 

 usually I to f as much in width. Their shape varies greatly : — sub- 

 angular, round, lenticular, illiptical, greatly elongated with rounded 

 ends and straight or gently curved sides, one side curved and the other 

 irregular or deeply embayed by prongs of granite, or entirely irregular 

 and in\'aded by tongues of granite. With the darker, are many lighter 

 colored xenoliths, porphyritic, but with a more finely granitoid ground- 

 mass, in some cases conspicuously sprinkled with minute hornblende 

 grains. These form separate masses and also sharply separated zones 

 surrounding the darker xenoliths, or attached to them on one or two 

 sides. An occasional mass of fine granite, of the same type as that 

 found as larger masses only a short distance away, may also be 

 found. The contacts of all of the xenoliths with the granite are sharp 

 though perfectly sealed. There is no evidence of textural blending nor 

 are there any reaction rims. The contacts of the lighter colored and 

 more granular xenoliths, while likewise sharp, are naturally not so 

 strongly marked to the eye as in the case of their darker companions. 

 Examination with a lense shows that the contact is not, however, a 

 simple line, but is indented by grains of the surrounding material and 

 sometimes by small apophyses. 



The darker xenoliths are here again strikingly similar in appearance 

 to certain phases of the rhombenporphyry, and are composed of a very 



