172 PROCEEDINGS OF THE ACADEMY OF [May, 



washing away of material from the solid rocks originally occupying 

 the ground. 



The cause of the ring of the boulders when struck is no doubt to be 

 sought in the peculiar rock texture. As seen in thin sections (fig. 5, 

 Plate II), the feldspar crystals interlace to form a close network in 

 which sound waves can readily develop. But this alone is not 

 sufficient — the boulders must also be supported so as to be able to 

 vibrate freely, just as is necessary with a bell, for thin sections of 

 two of them found side by side, one ringing finely, the other not at all, 

 show absolutely no textural difference (figs. 5 and 6, Plate II), but 

 the former was loosely supported, and the latter wedged firmly, 

 between other blocks. 



It is to be concluded, then, that the "ringing-rocks" consist of 

 boulders formed in the places where they are found by simple, normal 

 processes of weathering and that their ringing qualities are due to 

 the texture of the diabase rock of which they are composed. 



Explanation of Plate II. 



Fig. 1. — Sun-crack-like structure, upper surface of diabase sill, north of Neiffer 



P. O., Montgomery County, Pa. 

 Fig. 2. — Boulder of diabase showing exfoliation cracks, near summit of Spring 



Mount, Montgomery County, Pa. 

 Fig. 3. — Thin section of the rock of fig. 1, showing fine-grained diabase traversed 



by more coarsely crystallized dike. Crossed nicols. X 20. 

 Fig. 4. — Thin section beneath an exfoliation crack, showing uniform size of 



grain, the lighter color along the crack being due to decomposition of 



augite, etc. Ordinary light. X 20. 

 Fig. .5. — Thin section of a rock yielding fine ring, Ringing Rocks Park. The 



banded crystals are plagioclase feldspar, the gray patches chiefly augite. 



Crossed nicols. X 20. 

 Fig. 6. — Same, from a boulder adjacent to the preceding, which failed to ring; 



entirely similar as to minerals and structure. 

 Fig. 7. — Boulder field, Ringing Rocks Park, northeast of Pottstown, Montgomery 



County. 

 Fig. 8. — Boulder formation in diabase, quarry at St. Peters, Chester County, Pa. 



