94 PROCEEDINGS OF THE ACADEMY OF [1898. 



many small blisters or former cavities filled with a still whiter ma- 

 terial than the honeycombed mass in general. The lighter part of 

 the mass contained but little magnetite, while the dark portion was 

 filled with this iron ore. As plutonic rocks do not possess any 

 amygdaloid texture, I have concluded that those found in the Potts- 

 town park are of volcanic origin, that they were once a fluid lava 

 and that the rock masses as they appear are the remnants of a col- 

 lapsed crater. 



Microscopic examination of thin sections gave the following re- 

 sult : — 



The infiltrated material in the former gas cavities appeared by 

 reflected light as white, opaque, granular masses, which could not 

 be resolved even when magnified to about three hundred diameters 

 into any distinct crystalline form. Nevertheless, that it possessed 

 crystalline structure is indicated by its aspect in polarized light. 

 I diagnose the material as feldspatic. With ordinary light and with 

 the aid of an Abbe condenser the rock mass showed a faint green 

 augite ; the particles, very finely divided, often appeared together 

 in groups, and these small greenish individuals having a greater 

 refractive index than the feldspar, were, in consequence, more con- 

 spicuous than the greater mass of the rock. In polarized light the 

 feldspar appeared to be essentially orthoclase, some sanidine being 

 also present. This volcanic rock may be compared with the amyg- 

 daloidal basalt of Schlachenwerth, near Carlsbad, Bohemia. 



A general description of the rocks on the eastern side of the hill 

 is also of special interest. The enormous size of some of the sections 

 of basaltic columns found there, some undoubtedly in situ, others 

 removed to alower level, is such as to cause surprise. These basalts 

 are mostly six-sided, and as the result of weathering, somewhat 

 rounded on their edges ; some are perfectly flat and level, others are 

 found with a concavity varying in depth, while some have been seen 

 witli a wavy surface. Some of the columns do not seem to split 

 transversely, but stretch their bulky length some twenty feet or more 

 along the ground. One has been named the Bullfrog by the natives. 

 The diameters of the six-sided sections of the basalt vary. I meas- 

 ured some which were ten, eleven or fifteen feet across. These 

 gigantic illustrations of mountain-making may indicate a peculiar 

 geologic condition wherein a long continued internal heat, or possi- 

 bly a slow cooling dependent upon external causes, prevented the 

 contraction of the general mass into smaller columns such as are so 



