ABSTRACTS OF PAPERS 



65 



Author's reply : Professor Quirke's observations on marlekor have been 

 made in an area somewhat remote from the one in which my own studies were 

 made, and it is possible that some of the specimens referred to by him may 

 have a different genetic history from that of the marlekor of the Ottawa dis- 

 trict. I am not prepared to explain just how the fissures in the interior of the 

 marlekor originated, but the clay pebbles which may be seen in abundance 

 along the Ottawa River in late summer certainly show no such features; 

 neither do any of the mud balls which have come under my notice along the 

 Bay of Fmidy estuaries possess any features which suggest in any way a 

 comparable origin for the marlekor. The best evidence, perhaps, that the 

 marlekor were produced by the same general agencies which are responsible 

 for the claystones occurring at a lower level in the clays, is found in the fact 

 that, like them, they are confined to that portion of the clays which lies within 

 a few feet of the surface. 



BOOM BEACH fISLE-AU-HAUl\ MAIXEJrA SEA-MILL 

 BY JOHN M. CLARKE 



(Abstract) 



A short beach with rocky floor and bounded at each end by rock cliffs, ex- 

 posed to the full force of the Atlantic, so confines wave action as to prevent 

 lateral dispersion of abrasive effects, with the result that all boulders consti- 

 tuting the beach are ultimately reduced to forms of perfect symmetry. 



Presented in full extemporaneously. 



Discussion 



Mr. Chester K. Wextworth : I was interested in Doctor Clarke's paper be- 

 cause of its bearing on the problem of the production of flat beach cobbles by 

 abrasion. The idea is extant in many text-books that these are produced by 

 the shoving of cobbles to and fro by the waves. The proof or disproof of this 

 view waits for quantitative studies, and I should like to receive suggestions 

 as to suitable localities for such studies. In particular, I wish to ask Doctor 

 Clarke if there are at Boom Beach any facts which indicate a flattening of 

 cobbles and boulders as abrasion proceeds. 



Mr. Carl O. Dunbar : While working along the western coast of Newfound- 

 land I was strongly impressed with the fact that the structure of the rock 

 exercises a controlling effect on the shape of the beach pebbles derived from 

 it. In places along this coast there are thin-bedded, sedimentary rocks asso- 

 ciated with dikes and flows of basalt, and the contrast in the prevailing shape 

 of the pebbles derived from each of these types of rocks is most striking. The 

 most homogeneous, igneous rock breaks down into fragments of subequal 

 dimensions, which under the attack of the waves become rounded into sub- 

 spherical forms. On the other hand, the banded sedimentaries break up into 

 flatfish slabs and chips that ultimately become very oblate spheroids or disks 

 of the shape of a watch, and although they take on the most perfectly rounded 

 symmetry never approach a spheroidal form. 



Eemarks were also made by Mr. Arthur Keith. 



V — Bull. Geol. Sue. A.m., Vul. .34, 1922 



