92 PROCEEDINGS OF THE ACADEMY OF [1898. 



Eastward from the upper plateau of the terrace is an open area 

 devoid of plant life. It is called by the people the " Great Ringers," 

 Plate II, in contradistinction to the " Little Ringers," which are 

 situated on east side of the park. The outcrop consists of rocks of 

 various sizes, from a hand specimen to those weighing thirty tons or 

 more. They are essentially gabbro-phonolite, as I stated in a com- 

 munication made several years ago to the Academy. I then an- 

 nounced the mineralogical composition of these rocks to be: 

 plagioclastic feldspar, sanidine, diallage, augite, magnetite; ferrite 

 also was occasionally present. These rocks are very tough and 

 close-grained. Even a thin section has a considerable cohesion and 

 does not break in boiling in spirits of turpentine, unless the outer 

 weathered gray crust be taken. This is brittle when thus tested. 



Plate I illustrates a small section of this singular rock formation. 

 The outcrop has precisely the appearance of what has been called an 

 Einsturzkrater 2 or collapsed crater. The crater in its present condi- 

 tion is but a fraction of what it was formerly. Its former horizon can 

 still be traced ; vegetation has gradually encroached upon it and, if 

 not prevented, will cover the entire tract which is now bare. The 

 outlines of volcanic craters are extremely irregular everywhere, 

 whether the walls are still standing or have fallen in. The contour 

 may be traced, however, in either case, the rocks themselves being 

 the guides. It can be shown that in the building up of the eleva- 

 tion, various phases of volcanic action must have taken place to ac- 

 complish that which ultimately resulted. It seems probable, as has 

 been suggested, I think by Geike, and even observed by him and 

 others, that the phonolites are formed on the upper neck of the vol- 

 cano, and that the basalts are liquid lava poured out from some 

 point along the side of the crater. Similar relations appear to have 

 existed in the building up of the Pottstown Hill. The metallic 

 sounds emitted from the rocks, when struck with a hammer, are of 

 great interest to visitors, and both superstition and poetry are in- 

 cited thereby. The phonolites of this locality however, lose their 

 ringing property when broken. It seems probable that this effect is 

 due to the production of innumerable cracks through the whole 

 mass, just as a metallic bell or a glass jar loses the property of 

 ringing when cracked. There are, however, some phonolites, as 

 those occurring north of Wilmington, Delaware, and on Haycock 



2 Carl Vogt's Geologie, Band II, page 330. 



