X-^^l ,„ —■. 



488 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



were not met with elsewhere in the conglomerate. Some of the inden- 

 tations were much deeper than they were broad, and hence could not 

 have been formed hj the pressure of one rounded pebble upon another. 

 Mr. Crosby claimed that certain Assuring in the pebbles occurred dur- 

 ing the time of the compression ; but these fissures were found to be 

 filled with vcm-crystals of quartz, which had suffered the same indenta- 

 tion as the pebble, so that it was impossible for both to have been 

 formed at the same time. The forms in question were attributed by 

 Dr. Wadsworth to glaciation, the rock being found to be smoothed on 

 both its north and south sides as well as in its depressions ; AvJiile sand 

 action later appeared to have given rise to some of the peculiar forms 

 observed,, either directly or by the modification of previously existing 

 surfaces. It was also pointed out by him that some pebbles had in- 

 deed been compressed and indented ; but that the forms exhibited in 

 such cases were different from the peculiar ones imder discussion ; the 

 indentations, fractures, etc. being such as would naturally take place 

 when a fragmental rock like quartzite was subjected to compression. It 

 was furthermore remarked, that no softening and plasticity could have 

 occurred, because the grains of quartz were round and intact, and not 

 flattened, as Mr. Crosby's view demanded. 



This subject of plasticity of the conglomerate pebbles was again under 

 discussion before the Boston Society of Natural History, Mr. Crosby 

 claiming that he found compressed forms below the original surface, 

 and attempting to explain them by saying that they were due to a lack 

 of cohesion amongst the sand grains in the quartzite. This ground had, 

 however, been already taken by Dr. Wadsworth for certain forms ob- 

 served ; and it will be noticed that it begs entirely the question of the 

 softening and plasticity claimed. A rock that has been softened by 

 metamorphic agencies, so as to have become plastic, must be so through- 

 out its entire piass. Mr. Crosby's view was similar to that which would 

 be taken in regard to sand, if it were to be said that it ran through a 

 hole by virtue of its plasticity. 



Mr. Crosby also tried to show that the pebbles of the Eellingham 

 conglomerate had been squeezed or pulled out into spindle-shaped 

 masses ; but it was evident to more than one who examined his speci- 

 mens, that he had confounded the adhering matrix with the pebbles 

 themselves, — a not uncommon mistake. (Proc. Best. Soc. Nat. Hist., 

 1879-80, XX., pp. 308-318, 3G8-378, 405.) 



Mr. W. W. Dodge gave, in 1881, a paper containing numerous details 

 of his observations in the vicinity of Boston. In this paper he noticed 



