1912.] NATURAL SCIENCES OF PHILADELPHIA. 171 



hood, many a fantastic theory having been advanced to account for 

 their formation. The principal localities of these ringing-rock fields 

 are: 



Narrows ville, Bucks County, on the edge of the plateau south of 

 the Delaware River, 3 miles east of Kintners ville. 



Shelly, Bucks County, 2\ miles northeast of the railroad station. 



Spring Mount, Montgomery County, east of the Perkiomen Creek, 

 one mile above Schwenksville. 



Ringing Rocks Park, 2 miles northeast of Pottstown. 



Blue-Rocks, 1 mile east of Elverson Station, Chester County. 



Although most of these places are rather widely known and are 

 frequently visited by outing parties, curiously enough no scientific 

 description of them appears to have ever been published. 



The manner of formation of the large rounded rock masses known 

 as boulders, as described in text-books of geology, is as follows: 

 The solid rocks at or near the earth's surface are travi rsed by numer- 

 ous cracks, or "joints," usually intersecting in such a way as to divide 

 them into roughly rectangular or rhomboidal blocks. Rain water, 

 percolating downward along these joints, attacks and gradually 

 decomposes the rock constituents, but since at the intersections of 

 planes the action can take place in several directions at once, edges 

 yield twice, and corners three times as rapidly as the flat surfaces, 

 and as a result the ultimate shape attained by the blocks is that of a 

 spheroid (fig. 8, Plate II). 



Boulders thus developed usually remain surrounded by weathered 

 rock fragments or the derived soil; but when running water finds its 

 way around them, this finer material may be removed, leaving them 

 exposed to view. Study of the above-mentioned occurrences has 

 shown that this explanation is adequate to account for their forma- 

 tion. The boulder fields are always found at or near the base of the 

 sheet of igneous rock, where the intensely metamorphosed underlying 

 sediments form an impervious pavement over which streams of water 

 flow; this can sometimes be heard, or even seen, through spaces 

 between the blocks. Their exact position is determined by local 

 conditions, such as the dip of the strata, which apparently must be 

 less than 30°, the configuration of the hills, which affects the flow of 

 the streams, and perhaps other factors. But the point calling for 

 particular emphasis, in view of the popular opinion about the matter, 

 is that there has been absolutely no " upheaval" or other violent 

 disturbance to bring the boulders together, but that they have, 

 quite to the contrary, been formed by the gradual breaking up and 



