ABSTRACTS OF PAPERS 85 



that they occur only within the range of the floods, usually only on the upper 

 surfaces of the cobbles, and on cobbles of undoubted local origin in a region 

 that has never been glaciated. If I am not mistaken, I liave even seen articles 

 of modern human manufacture that bear such marks. Such cobbles after 

 being faceted on one side are sometimes turned over and acquire other facets. 

 They then bear a very marked resemblance to glacial pebbles and are indis- 

 tinguishable from the faceted cobbles from the Potomac Valley which Pro- 

 fessor Hobbs believes may be derived from glacial Newark conglomerates. I 

 believe that such effects may be produced on the banks of any river that has 

 occasional ice jams. The grinding of the large volume of ice that passed down 

 the Potomac in March, 1918, in my opinion, may liave been sufficient to pro- 

 duce such marks, and such ice jams if repeated many times would certainly 

 do so, for the cobbles embedded at the surface of the mud, especially if the 

 mud were frozen, would remain in place and receive repeated scourings. 



Facets and strise can also- be made without ice action of any kind. If a 

 conglomerate that is not so strongly cemented that it will break across rather 

 than around the pebbles is sheared, the pebbles will grind against each other 

 and many of them will receive facets and strife that are indistinguishable 

 from those of glacial origin. Of course, the movements may be so localized 

 that ordinary slickensides will result. But if the conglomerate is but weakly 

 cemented, or if it is in a position approaching the zone of flowage, the move- 

 ment may be distributed throughout its mass, so that many faceted pebbles 

 may result, or even the whole conglomerate may be so thoroughly ground up 

 that it resembles a tillite. I believe that many supposed "tillites" have this 

 origin. Some of the early Tertiary conglomerates in the Matanuska Valley, 

 Alaska, contain abundant faceted and striated pebbles that apparently are as 

 characteristically of glacial origin as the recent glacial pebbles of the same 

 region, yet these conglomerates are interbedded with rocks that contain warm- 

 climate plants, including even palms. I broke open one of these pebbles, which 

 proved to have been slightly sheared, and found that the "glacial stride" inside 

 the pebble were just like those on the outside. 



Mr. David White : In his parallel between the Atlantic Trias and the di- 

 visions of the Indian Gondwana, Doctor Hobbs seems to have given great 

 weight to the points of physical resemblance. The statement as to the paleon- 

 tology of the Deep River basin appears to constitute corroboration from an 

 outside source. I fear, however, that Doctor Hobbs has given too ready ac- 

 ceptance to a Paleozoic reference of the Deep River beds, which should not be 

 classed as Permian before the fossils are passed on by. the appropriate special- 

 ist or specialists. 



QUANTITATIVE CRITERIA IN PALEOGEOGRAPHY 

 BY RAYMOND C. MOORE 



(Ahstract) 



Volumetric determinations of geological formations afford information, in 

 certain instances, of paleogeographic significance ; but, due to the <iuantita- 

 tively unknown values of various components, definitely detailed conclusions 

 are not ordinarily possible. Concerning assumed distribution of sea and land 



