27G PROCEEDINGS OF THE AMERICAN ACADEMY. 



more distinctly crystalline groimdmass and, on the other, there are 

 gradations into fine grained types with few or no feldspar phenocrysts. 

 So varied, in fact, are these xenoliths that detailed description is out 

 of the question and only certain significant features can be noted. 

 Where feldspar phenocrysts occur they are always characterized by 

 a central core of more or less acute habit about which is a later rim 

 usually, if not always, showing inclusions of the groundmass grains. 

 In the more basic xenoliths the core is in part, if not entirely, homo- 

 geneous, or it is finely cryptoperthitic; in the more acid types the 

 core is more distinctly perthitic, often wholly so. The quartz pheno- 

 crysts, which occur sparingly in the darker types, are always mai'ked 

 by a more or less strongly developed rim of pyroxene or hornblende; 

 in the lighter colored and more siliceous types the quartz is more 

 abundant and the margin of dark silicates is less marked or nearly 

 wanting. In the more acid types, while some augite is present, it 

 shows a tendency to pass into a green variety, and most of the pyro- 

 xene is distinctly green, slightly pleochroic, and seems to be an aegi- 

 rine-bearing augite probably near augite, since the double-refraction is 

 always much too low for true aegirine-augite. It is of earlier age 

 than the hornblende, which in this type l)ecomes rather abundant and 

 grows a])out the pyroxene in part, and in part, occurs separately, 

 enclosing poikolitically the feldspar of the groundmass. Most of the 

 hornblende appears to be related to the catoforites although it is in 

 part riebeckitic, particularly about the margins. The latter is also 

 disseminated as shreds and fibers through the rock. Magnetite is 

 an abundant alteration product, accompanied by some biotite. It 

 may be noted here that some xenoliths occur in which little or no 

 pyroxene is present, its place being taken by hornblende, apparently 

 original, although it is true that all the xenoliths of this type that 

 have come under the writer's observation are quite heavily altered, 

 and it is possible that a part of the hornblende may be secondary 

 after pyroxene. Apatite is common in the more basic types, less com- 

 mon in the others. Titanite is present in all, forming irregular masses 

 sometimes associated with the hornblende, but it often lies between 

 or wrapped about the groundmass feldspar and is believed to be 

 wholly of secondary origin. 



In the acid types fluorite is present in small grains with the dark 

 minerals and also, like the titanite, wrapped about the feldspar. A 

 little zircon is also present. In most of the xenoliths examined, 

 alteration has produced considerable calcite, kaolin and other 

 products. 



