432 JULIEN 



itic rock (Potsdam). The close correspondence to the succes- 

 sion on Manhattan Island is apparent. 



These beds have been invaded by an extensive group of inti- 

 mately related igneous rocks, presenting the following facies ; ^ 

 primary hypersthene gabbro, often with brown dichroic horn- 

 blende, passing into gabbro-diorite, hornblende-gneiss and horn- 

 blende-schist ; into gabbro-granite, by addition of biotite ; into 

 norite, with excess of feldspar and quartz ; into gabbro-diorite, 

 by paramorphism of pyroxene to green hornblende ; into gab- 

 bro-diorite and hornblende-gneiss, by addition of green horn- 

 blende, and this into epidotic diorite, by epidotic alteration of 

 feldspar ; an intrusive serpentine also occurs. 



In regard to the distribution of quartz, it was noted that in 

 the gray trap, " the rock is composed of plagioclase-feldspar and 

 hornblende, with frequently a small proportion of blue quartz 

 and biotite. Massive hypersthene often entirely replaces the 

 hornblende." . . . The rock thus ranges from a quartz-diorite 

 to a true hyperite. "■ The chief distinguishing feature of the 

 Delaware gabbros is their highly acidic character, due to the 

 large admixture of quartz." The hypersthene-gabbro, or so- 

 called " blue granite," ** is normally a finely granular mixture 

 of basic plagioclase, hypersthene and diallage in nearly equal pro- 

 portions, with accessory quartz, magnetite, apatite, hornblende 

 and biotite. . . . The rock is a most unstable form, running 

 on the one hand into a rock possessing strongly the character 

 of a granite and on the other into those distinctly dioritic." 

 " That these massive rocks have been subjected to great 

 pressure is shown by mechanic deformations in a crushing of 

 the quartz and in an occasional bending of the mica laminae.'' 

 The gabbro granites show" all degrees of semi-foliation " through 

 increase of biotite with probably accompanying pressure," with 

 production ofyf^j-^r-structure and passage into schistose gabbro- 

 diorites and hornblende-gneisses. The rocks described are of 

 the same general character as in Maryland and offer the same 

 genetic relationship, but under pressure-conditions which ap- 

 proached more closely than at Baltimore to those which pre- 

 vailed on Manhattan Island. 



1 F. D. Chester, U. S. Geol. Surv., Bull. No. 59, 1890, 1-43. 



