1879.} 313 [Crosby. - 
possible to connect them with either the jointing of the rock or the 
transverse fissures of the individual pebbles. It is, of course, possi- 
ble to break these materials conchoidally ; but the smoothness and 
uniform direction of the concave surfaces, and the fact that they are 
not limited to the present surface of the conglomerate, appear to me 
insuperable objections to the fracture theory. I accept the compres- 
sion theory, not as entirely satisfactory, but as beset with fewer ditffi- 
culties than any other. The numerous fractured pebbles show that 
the degree of plasticity has not been great; and in some cases the 
distortion of the pebbles is clearly the combined result of fracture 
and plasticity. Some plasticity is certainly required in those cases, 
of which I have observed several in this ledge, where a fracture pro- 
ducing a fissure one-eighth or even one-fourth of an inch wide on 
one side of a pebble terminates before reaching the other side. 
There are many points in the Boston basin where the hard, fine, 
quartzite pebbles constitute the main part of the conglomerate; but 
I have never observed the curiously indented pebbles outside of the 
ledge in Brighton here described, although since these were first 
discovered I have searched for similar phenomena in scores of ledges 
in various parts of the basin. 
Dr. M. E. Wadsworth, in criticism of the views held in 
the preceding paper, said: 
The question presented to us this evening by Mr. Crosby, has 
been one of much interest to this Society and to American geologists. 
The subject was first brought before the Society on the evening of 
January 4, 1860, by Edward Hitchcock, Jr., at which time he 
claimed that the pebbles in the conglomerates at Newport, R. I. and 
at East Wallingford and Fairfax, Vt., had been distorted and elon- 
- gated while in a plastic state after deposition. This view was opposed 
by Dr. Chas. T. Jackson, who contended that similar forms could be 
seen any day on our beaches. (Proce. Bost. Soc. Nat. Hist., vir, 209.) 
In October Professor Edward Hitchcock brought the subject again be- 
fore the Society (Proc. vir, 353), and was opposed by Dr. Jackson. 
Prior to this at the Newport meeting (August, 1860. Proc. A. A. A.S. 
XIV,112), ofthe American Association for the Advancement of Science, 
Prof. Charles H. Hitchcock read a paper on the ‘‘ Geology of the 
Island of Aquidneck,” in which he held that the Newport conglomer- 
ate had been in a plastic or semi-plastic state since its consolidation, 
and that the contorton and elongation of the pebbles took place then. 
