30 



INDENTED PEBBLES. 



FIG. 6. 



Fio. 7. 



in two directions. 5. They are cut across by parallel joints or fissures, varying in distance 

 from each other from one or two inches to many feet. The most distinct of these joints, 

 which are a rod or two apart, are perpendicular to the horizon, and nearly at right angles 

 to the strike, and make a clean cut from top to bottom of hills thirty or forty feet high. 

 Abrading agencies have often removed the rock on one side of these joints, or between two 

 of them, so as to leave walls of pebbles smoothly cut in two ; the whole appearing like a 

 pile of wood that has been neatly sawed. Acres of such walls may be seen in the vicinity 

 of Purgatory. Often the surface of the pebbles, thus cut through, is not only perfectly 

 even but smooth, and seemingly polished. Yet the two parts of the pebbles thus cut off 

 perfectly correspond, and one part has never been made to slip over the other. In some 

 minor joints single pebbles are not entirely cut off, but are sometimes drawn out of 

 their beds at one end when the rock is separated, and remain projecting above the 

 general cleaved surface. These joints, also, do not always extend through the whole 

 rock. We would be glad to introduce here many sketches of specimens illustrating these 



statements. But one or two must 

 suffice. Fig. 6 will give some idea 

 of an elongated pebble from Newport, 

 which is 10 inches long and 3 inches 

 across its broadest part. 



Fig. 7 shows a pebble 8 inches long 

 with a deep indentation. 



Perhaps I ought to add that some- 

 times the elongated pebbles partially 

 or wholly lose their rounded form at 

 the ends, begin to assume a foliated 

 . or schistose aspect, and are some- 

 what blended with the talcose or 

 micaceous cement. This, however, 

 is not generally though not unfre- 

 quently the case. 

 From these facts we could hardly avoid drawing the following conclusions : 



1. This rock was once a conglomerate of the usual character, except in the great abun- 

 dance of the pebbles, and it has subsequently experienced great metamorphosis, making 

 the cement crystalline and schistose, and elongating and flattening the pebbles. 



2. The pebbles must have been in a state more or less plastic when they were elongated, 

 flattened and bent. If their shape has been thus altered, their plasticity must of course 

 be admitted : for the attempt to change their present form would result only in fracture 

 and comminution. The degree of plasticity, however, must have varied considerably; for 

 some of them are scarcely flattened or elongated at all ; and, as here stated, some are not 

 cut off by the joints.' 



The neat and clean manner in which the pebbles have been generally severed by the 

 joints, implies plasticity. For though occasionally we meet with one that has a somewhat 

 uneven surface, as if mechanically broken, such cases are rare. Whatever may be our 

 theory of the agency that has formed the joints, the conviction is forced upon every 



