86 



Hyde Park is substantially a repetition of that studied on 

 Marblehead Neck and at other points. The jaspery patches 

 and quartzose axes or nuclei, however, have not been distinctly 

 observed elsewhere, and in these, it seems to me, we have 

 another strong argument against the hypothesis that the lenticu- 

 lar layers are flattened pebbles. Such uniformity of composi- 

 tion and structure in the pebbles of a conglomerate rock would 

 be something marvellous, but still more unique must the parent 

 rock have been, from which such quartz-centered, feldspathic 

 pebbles could be derived. I have already stated that the 

 separation of the quartzose and feldspathic materials is the result 

 of a segregating process. This is sufficiently proved in the case 

 of the axial layers, by the various degrees of distinctness which 

 they exhibit, and very conclusively in the case of the jaspery 

 patches, by the fact that undoubted pebbles are penetrated, or 

 cut entirely across, by narrow threads or veins of the red, jas- 

 pery material, which are sometimes connected with adjoining 

 patches or masses of the same; i.e., are prolongations of 

 them. In one case (PI. 2, fig. 3) a well-defined triangular frag- 

 ment of a greenish diorite is neatly divided by a narrow vein of 

 jasper, which penetrates the petrosilex on either side an inch or 

 more. Such examples, while proving the origin of the jasper 

 by segregation, also show that the segregation has been effected, 

 partly, at least, since the enclosure of the pebbles ; but not 

 wholly, for some of the petrosilex pebbles themselves enclose 

 isolated grains and small masses of jasper. 



Those believing in the pebble origin of the lenticular layers will 

 probably suggest that the segregation of the quartz may have 

 gone on simultaneously with the flattening of the pebbles, the 

 plasticity which permitted the latter favoring the former. The 

 principal answer here must be the uniform character of the layers. 

 One thing is certain, — the unflattened, angular pebbles or frag- 

 ments in the rock cannot be appealed to as evidence in favor of 

 the flattened pebble theory, for it is perfectly clear that the petro- 

 silex fragments post-date the lenticular layers. These fragments 

 have originated in this rock subsequent to its formation and subse- 



