196 



evidence in the ledge of felsite at the western base of this hill 

 and some 500 or GOO feet south of Lincoln Street. At this 

 point, however, the felsite, although still of a reddish or 

 purplish color, is nuich more compact and homogeneous. 



Although there is no o})])ortunity to observe its relations to 

 other ro(;ks, the effusive or volcanic nature of the red felsite 

 forming the numerous bowlders east of Squirrel Hill and the 

 solitary ledge already referred to, on Downer Avenue, is 

 abundantly proved by its structural features. It exhibits 

 throughout a distinct but not conspicuous banding or flow- 

 structure, which is usually rather fine, but sometimes quite 

 coarse and often somewhat contorted, discontinuous, or other- 

 wise irreo-ular. The confused and irreo;ular ciiaracter of much 

 of the bandinii' is due in part to the enclosure of anjiular fraii- 

 ments of the same or a very similar felsite. The enclosed 

 fragments vary from a small fraction of an inch to several inches 

 in diameter, and are very irregularly distributed, so that while 

 the greater part of the felsite is comparatively free from them, 

 they vary in the remainder from thinly scattering to densel}' 

 crowded, occasional masses of the rock being packed so full of 

 fragments that the banding is completely obliterated and it 

 closely resembles an ordinary breccia. This fragmental character 

 of the felsite has su"(j::ested to several observers, includin<; the 

 present writer, that it is a metamorphic conglomerate. But 

 having ol)served precisely similar structures (banding and 

 brecciation) in modern obsidian, I have been for a long time 

 thoroughly convinced that this metamorphic theory is untenable, 

 and that the felsite is a true volcanic rock, a devitrified ol)sidian. 

 Among the arginnents against its sedimentary origin arc the 

 facts that the fragments are all of the same kind of rock ; that 

 they arc ne\er assorted or show in any way the action of water ; 

 and that the felsite appears, as has been proved by analyses in 

 other cases, to be chemically essentially intact and homogeneous, 

 still retaining in e\ery j)art the full proportion of alkalies 

 requiriid for an acid felds][)ar, which would be very unusual in a 



