OCCLUSION OF IGNEOUS ROCK 483 



At the crossing of the broadest part of this belt of massive 

 gabbros and hornblendic derivatives by Brandywine and Chris- 

 tiana creeks, the city of Wilmington is situated. 



Pennsylvania. — A still closer correspondence in metamorphic 

 forms and structures of old igneous occlusions is met in a tract 

 of crystallines which lies still nearer to Manhattan Island. In 

 the vicinity of Philadelphia crystalline gneisses and schists occur, 

 attributed to alteration of Cambro-Silurian sediments, and are in 

 large part identical with those of Manhattan Island.^ A similar 

 succession has also been determined, the highest stratum being 

 micaceous, comparatively soft and very schistose ; this overlies 

 a crystalline limestone ; and beneath follows a hard and more 

 quartzose gneiss. - 



These schists are penetrated by many igneous occlusions, in 

 part of more or less basic type, like those whose former existence 

 on Manhattan Island has been inferred from the derivative schists. 

 For example, on the Chester Creek section, gabbro and pyrox- 

 enite (Rand, p. 291) ; "near Aldham, a gabbro in a very distinct 

 dike about 100 feet wide, with porphyritic feldspar crystals" ; 

 dikes of norite (Rand, p. 327) ; in Willistown township, massive 

 enstatite or bronzite (Rand, p. 311), etc. Analyses of diorite 

 and hornblende slate, as well as of hornblende picked out from 

 these rocks ^ approximate the results obtained on corresponding 

 specimens from Manhattan Island.^ With these intermediate 

 or basic types of igneous rocks, acid varieties are associated in 

 the form of pegmatite, aplite and granulite, and it has been 

 affirmed : " All the rocks of this belt can be traced back to an 

 original pyroxenic magma erupted through the azoic schists 

 which surround the belt." '^ 



The alteration products, however, of these definite igneous 

 types are more common and closely resemble those on Man- 



'J. D. Dana, Am. Jour. Sa'., (3), xxi, 1881, 439; and F. J. H. Merrill, idem, 

 (3), xxxix, 1890, 3S7. 



2T. D. Rand, Pror. Acad. Xat. Sci. PJiila., 1900, 221. 



3 A. G. Leonard, loc. cit., 141, 144, 146. 



^Julien, loc. cit., 439, 468. 



5F, D. Chester, Second Geol. Surv. Penn., Ann. Rep. 1887, 105. 



