SCHISTS OF THE APPALACHIAN BELT 486 



These facts alone seem to me decisive on the question of the genesis of 

 the latter. 



A PPA LA CHI A N BEL T 



Man\^ of the hornblende schists which occur in al)undance along our 

 eastern coast must be closely allied to that of Manhattan island. In 

 New Hampshire the origin of a hornblende schist has been attributed 

 to alteration of a diorite. ^ In the collection at Columbia University a 

 specimen of hornblende slate from Woodbury, Connecticut, is identical 

 with that of this island. A specimen from Pennsylvania of finely fibrous 

 black hornblende schist of silky luster (specific gravity, 3.027), labeled 

 " West side of the Schuylkill river, 2 miles north of Girard avenue, Phila- 

 delphia," closely resembles that of our locality at West One hundred and 

 thirty-fifth street, with the same characteristic crumbling texture. In a 

 letter dated May 1, 1899, from m}^ friend, Mr Theo. D. Rand, recent!}^ 

 deceased, of Radnor, Pennsylvania, he stated concerning outcrops in that 

 vicinity : '' We have some interesting diabases, diorites altered into horn- 

 blende schists (dikes) and gabbros." Doubtless the series of crystalline 

 rocks in vicinity of Philadelphia includes the same hornblende schist 

 whose genesis is under consideration. 



Similar schists are distributed further south along the Appalachian 

 belt, through Virginia, the Carolinas, and Georgia; but one tract north- 

 west of Baltimore, carefull}^ studied by G. H. Williams,t offers some 

 important points of difi'erence which may intimate the initial history in 

 development of our own schist. There a h3q)ersthene gabbro ("the 

 original type ") passes into a greenish gabbro diorite, massive or schistose 

 (analysis XXIII). The latter form, described as anorthite amphibolite, 

 diorite schist, or amphibole schist, was composed of fibrous green horn- 

 blende, possessing a satiny luster, recognized as " the paramorphic pro- 

 duct of the pyroxene," and of opaque white feldspar, anorthite. A speci- 

 men in the collection at Columbia University of the laminated horn- 

 blende schist from White's granite quarry at Baltimore is identical in 

 satiny luster and general character with that of Manhattan island and 

 of New Rochelle, New York. The chief differences in constitution of 

 these diorite schists and our own are interesting and significant. 



First. There is a general abundance of feldspar, anorthite ; somewhat 

 inferior specific gravity, 2.996 to 3.069 ; and passage into pN^'oxenic rock. 



Second. There is a rarity of quartz, that mineral occurring only as 

 minute inclusions and in veins associated with granite and pegmatite. 



* G. W. Hawes, Geol. of N. H ,vol. iii, pt. iv, 1878, p. 231. 



tU. S. Geol. Survey, Bull. no. 28, 1886, p. 49; also Johns Hop. Univ. Circ, no. ao, 1884. 



LXVII— BxJLL. Geol. Soc. Am., Vol. 14. 1902 



