Rogers.] 100 [May 19, 



some cases, by an actual turning of the pebbles from their originally 

 flat position by the oblique action of the upheaving force. 



A striking feature in the general structure of these rocks, is the 

 system of vertical joints by which they are traversed, and -which 

 have often been alluded to by former observers. These joints, rang- 

 ing nearly east and west, or at right angles to the strike of the beds, 

 are usually at distances of twelve to fifteen feet apart, but in some 

 cases they divide this interval by parallel clefts only a few inches 

 asunder. AYhcre this is the case the wealing action of the waves 

 finds comparatively little opposition, and the cliff in process of time 

 is cut back, so as to form a chasm of greater or less length, whose 

 vertical parallel sides extend from the top of the cliff to its base. 

 Of these effects of erosion, one of the most striking is the well-known 

 chasm at Purgatory, near Newport, which has been erroneously re- 

 garded as due to the decay of a dyke of trap, supposed to have 

 occupied the cavity. 



As already stated, the above objections to the plastic theory are 

 meant to apply simply to the mass known as the Newport conglom- 

 erate, having its typical locality in the Purgatory rocks, and are not 

 intended to throw doubt on the evidences of metainorphie action, 

 mechanical and chemical, with which, in other cases, geologists are 

 familiar. Of the reality of former movements within the substance 

 of rocky strata we have abundant illustration in the actions by which 

 slaty cleavage has been induced, and by which, in connection with 

 this structure, the lengthening, shortening, and other distortions of 

 the enclosed fossils have been brought about. These distortions, 

 however, in most cases, are to be explained not so much by a direct 

 compressing or extending force, as by the effect of the sliding of the 

 laminae upon each other in definite directions, carrying with them the 

 corresponding linear elements of the fossil, or its impression ; so that 

 without any necessary coudensaiion or stretching of the mass, the 

 distorted forms may be regarded as so many geometrical projections 

 of the fossil on ditfereutly inclined planes. 



In recent explorations of the conglomerate, I have obtained im- 

 pressions which, although indistinct, are suggestive of the " Lingula," 

 loan 1 many years ago in the conglomerate rock in the neighborhood 

 of Fall Hiver, a deposit probably on the same, or nearly the same, 

 geological horizon with the Newport conglomerate. Besides these 

 specimens, which were broken from the rock in place, I have found 

 numerous large pebbles on the adjoining beach crowded with well- 

 preserved impressions of the same fossiL These pebbles, both in 



