1893.] NATURAL SCIENCES OF PHILADELPHIA. 177 



thickness. When struck with a hammer it gives a clear metallic 

 sound ; even the knuckles or the finger nails produce an audible 

 ring. It is fine grained, compact, tough, rough to the touch and of 

 dark gray color ; not weathered. 



The thin section exhibits under ordinary light much colorless 

 material with some dark, irregularly outlined patches. Dichroism 

 hardly any. Between the crossed nicols the following minerals 

 became visible : Plagioclastic feldspar, sanidine, augite, diallage, 

 amphibole, ilmenite with borders of leucoxene. Glass was not 

 found. 



Similar thin slabs of the same material were found in two other 

 localities. The one near the dyke at Schwenksville near Perkiomen 

 Creek and the other on the dyke near Tylersport, Montgomery County, 

 Pa. Both slabs were about i inch thick and weathered. The mere 

 touch with the fingers emitted a sound. The mineral composition in 

 general is the same as the rock from Little French Creek Falls. 



An Altered Phonolite. A flat slab, of about a 1 inch in 

 thickness, having the appearance of micaceous gneiss and a decided 

 metallic sound when touched or struck was found not far from the 

 dyke at West Conshohocken, Montgomery County, Pa. A thin 

 section observed under the microscope indicated a glassy feldspar 

 having the characters of sanidine. This latter species was compared 

 with the sanidine bearing trachyte from Sicily and Vesuvius and 

 seems to be identical therewith. Besides this glassy orthoclastic 

 feldspar a mica and quartz are present. There are also some very 

 thin fibrous microlites, which are probably a silicate, and ferric oxid. 

 In the whole the mixture of minerals present in this peculiar slab is 

 fine grained and very brittle. If the glassy feldspar should prove 

 to be true sanidine, the rock would be a new variety of gneiss, and, 

 since sanidine occurs only in eruptive rocks a somewhat singular 

 derivation would be indicated. It is evident that the glassy feld- 

 spar resists the action of metamorphic influences longer than the 

 plagioclastic feldspars ; the latter having entirely disappeared in this 

 altered phonolite, being, as it seems,changed into mica and quartz. 

 This is believed to be possible, since labradorite contains about 53 

 per cent, of silica, muscovite 46 per cent. The difference of the silica 

 would be liberated and, in the course of time crystallize into quartz. 



Gabbro-Phonolite. About ten miles east of Quakertown, 

 Bucks County, Pa., a conical hill rises, named by the people, Haycock 

 Mountain. On the slope, near the top of this hill, is an outcrop of 



