398 JULIEN 



weakness that the massive portions of the rock are often 

 traversed by white quartz in minute seams, rarely over 0.5 

 millimeter in thickness, which weather out in delicate, often 

 reticulating ridges. These plainly denote fine cracks produced 

 by shattering of a brittle mass. 



Pegmatite veins or dikes have also insinuated themselves 

 along the same planes in large numbers, parallel or branching 

 and crossing, generally less than 5 centimeters in width, reaching 

 40 centimeters in one case. Their material consists chiefly of 

 white feldspar, grayish quartz and a little white muscovite, crys- 

 tallized along the central plane of the sheet, but sometimes only 

 of milky quartz. 



It thus appears that the series of three successive igneous in- 

 trusions is well illustrated in this bed. Their coincidence at 

 many points is so well displayed that one may cover the con- 

 tact of all three — diorite, lens of earlier pegmatite and pegma- 

 tite dike of the last intrusion — with the palm of one's hand. 



Evidence of Lhicoiifonnity. — The structure just described in 

 these intrusions of basic igneous rocks during Paleozoic time - 

 — their intercalation parallel to bedding or foliation of the 

 altered sediments they have entered — is a remarkable feature 

 which prevails throughout the Appalachian belt. It may of 

 course but conform to the suggestive observation in another 

 region that " in such cases the dykes were probably not very 

 different in age from the gneiss which they traverse " ^ — a rela- 

 tion which, if established, may have been connected with inferior 

 superincumbent pressure or with imperfect consolidation of the 

 invaded beds. In one neighboring locality, indeed, they are 

 found in dykes, distinctly intersecting other intrusions or the 

 layers of the associated country rock, viz., those of the Cort- 

 landt Series near Peekskill, New York. 



Elsewhere the published evidence on this problem seems to 

 be indefinite and insufficient. As to Pennsylvania we have the 

 statement, '* a long narrow belt of sphene-bearing amphibolite 

 schist in the city of Philadelphia . . . cuts across the meta- 



1 J. D. Dana, Am. Jour. Sci., (3), XXVIII, 1884, 386. 



2 Bauerman, Quart. Jour. Geo/. Soc, XIA, 1885, 144. 



