278 PROCEEDINGS OF THE AMERICAN ACADEMY. 



prevalence of xenoliths in the coarse granite of the quarries has been 

 noted above. Mr. Dale *^ states that their sizes range from one 

 half an inch to 2 feet by 1 foot, 6 inches; 2 feet, 6 inches by 2 feet, 

 6 inches; 3 feet by 4 inches, and 6 feet by 2 feet, but that they 

 are usually small and roundish or elliptical in outline. The present 

 writer is quite in accord with these statements. Mr. Dale also 

 divides the "segregations" into three classes, which so far as their 

 megascopic characters are concerned are substantially the divisions 

 given below. 



Although there appears to be more or less gradation and no sharp 

 line can be drawn, three types may be made as follows: — 



(1) Essentially fine-grained to almost dense xenoliths of a dark to 

 medium bluish or greenish-grey color; usually irregularly, though not 

 abundantly porphyritic, the phenocrysts being chiefly feldspar and 

 quartz with occasionally irregular patches of p;^TOxene or hornblende. 

 The phenocrysts are sometimes rudely clustered. 



(2) Essentially fine to medium grained xenoliths of light to medium 

 grey color: — about the same shade as the enclosing granite; usually 

 contain a few, sometimes a good many phenocrysts of feldspar and 

 quartz and occasionally irregular crystals of pyroxene or hornblende. 

 The phenocrysts are not as a rule very evenly distributed and tend to 

 form clusters. 



(3) Fine-grained greenish or yellowish-green xenoliths often having 

 a feeble banding; not usually porphyritic. These are substantially 

 like many of type (1), and appear to diifer from them chiefly in being 

 more altered and in having been sometimes sheared, thus developing a 

 banded structure. 



The contacts of these xenoliths is essentially a sharp one, though 

 naturally the grains of the surrounding granite project into the 

 xenoliths about the margin and occasionally tongues of the granite 

 penetrate them. There is never any sign of chemical interreaction be- 

 tween the two. 



Thin sections of the darker colored xenoliths of type (1), show that 

 they are essentially a pretty even-grained mixture of microcline- 

 microperthite and some quartz, aegirite-augite, aegirite and green or 

 blue alkali-hornblende. The latter and a part of the pyroxene often 

 encloses the feldspar in poikilitic fashion. With these are the usual 

 accessory minerals found in the granite. The lighter colored xeno- 

 liths of this type (1) are more sihceous and contain proportionally 



43 loc. cit., p. 96. 



