1884.] NATURAL SCIENCES OP PHILADELPHIA. 243 



northeast, outcropping upon the run to the south of Pleasant Hill 

 P. 0. Many other instances of granite veins might be cited, but 

 with nothing new regarding them. All of the large veins men- 

 tioned above are exclusive of the smaller seams, which vary in 

 width from a few inches to a foot, and which are liable to be found 

 anywhere, and frequently, within the micaceous belt. 



The second class of granite — a highly changed gneiss or schist — 

 is a fine-grained rock containing quartz, plagioclase and biotite, 

 with the quartz subject to considerable variation. 



It occurs as massive beds, the planes of the stratification being 

 so completely obliterated that the rock resembles a true trap. 

 That it is not trappean, however, is shown by the fact that it is 

 seen to run, by indistinct degrees, into mica schist. Such rocks ai'e 

 usually much broken up, thus testifying to an undue mechanical 

 activity, itself the cause of the extreme metamorphism. 



Granitic Gneiss and Mica Schist. — These two species represent 

 the extremes of variation in what is the characteristic rock of the 

 micaceous belt, which has gone by the generally applicable name 

 of gneiss. These two rocks so merge into each other that specific 

 designation is often difficult. The typical mica schist may be 

 described as a very schistose biotite rock, usually highly garnet- 

 iferous, and containing a variable proportion of quartz. Some- 

 times this highly micaceous rock contains a very small proportion 

 of feldspar, which can often only be seen as a kaoline substance 

 in the decomposed product ; and if the absence of feldspar be 

 characteristic of mica schist, then with the presence of feldspar 

 the true schists begin to run into gneiss. 



Hornblende Bocks. — The hornblende rocks, as a class, may be 

 divided into the basic and acidic, or into those rocks of which the 

 predominating constituent is either hornblende or feldspar. To 

 the former belong the dark varieties of amphibolite schist and 

 syenitic gneiss, and to the latter belongs the light, highly acidic 

 bluish gray trap so characteristic of the northeastern part of the 

 State. Between these two extremes there is every shade of 

 gradation, showing some petrographical relation between them. 

 The dark varieties of amphibolite schist vary in color from a blue 

 to a dull black, from coarsely crystalline to compact. The pre- 

 dominating element is hornblende, with which is associated a 

 small proportion of plagioclase, and sometimes blue quartz. This 

 rock, which shows a more or less eminent lamination, is found 



