OCCLUSION OF IGNEOUS ROCK 485 



enstatite rock) ; near Glen Riddle, some of the serpentine is 

 after actinolite (Rand, p. 316). " It was my aim to distinguish 

 the hard, nearly black serpentines, derived chiefly from enstatite 

 or bronzite, . . . from those of a lighter color, with less serpen- 

 tine proper and with talc, steatite and antholite . . . probably 

 altered peridotites in large part," containing pseudomorphs after 

 olivine (Rand, p. 304). Reference is made also to ** slaty 

 serpentine, very dark olive green, and sometimes almost black 

 in color," associated with schiller-spar and enstatite ; ^ or with 

 " asbestus, enstatite and quartz, the last resulting from the 

 alteration of the serpentine." " The talcose serpentinoid derived 

 from peridotite has been designated a perido-steatite.^ It is of 

 interest to note that the same variations prevail in the serpen- 

 tinoid at Philadelphia as at New York City, and that the two 

 distinct kinds, above referred to, are found in different parts of 

 the same Hoboken-Staten Island outcrop. 



At Philadelphia then, at the crossing of this belt of igneous 

 occlusions by the Schuylkill river, we find the location of 

 another great city. 



Massachusetts. — We owe to the investigation of numerous 

 local petrographers a good knowledge of the series of crystal- 

 line formations which make up the Boston basin, and many of 

 which display traces of the fluent structure of ancient volcanic 

 lavas, some of which have been extravasated upon the surface. 

 Dike structure is of very common occurrence. They comprise 

 an extensive series of acid eruptives, granite, felsite, petrosilex 

 and porphyries, as well as more or less basic exotics, norite, 

 diabase, diorite, hornblendite, hornblendic gneiss and other 

 metamorphic forms ; these have been in part attributed to 

 Huronian age. The correlation of these classes has been 

 established at several points in the basin, and their develop- 

 ment by differentiation from a common magma. The basic 

 varieties are most fully distributed on the north of the present 

 site of the city of Boston, though to less extent on the west and 

 south,'' where acid forms prevail. 



iRand, op. ciL, 1876, 3. 2 Rand, idem, 1880, 241, and 1890, 1 18. 



"Bascom, Proc. Acad. Nat. Sci. Fhila., 1 896, 219. 



*W. O. Crosby, Occl. Papers, Bost. Soc. Nat. Hist., Ill, 1880. 



