NEW YORK ACADEMY OF SCIENCES 599 



recently been discussed by Berkey, 1 who finds that the St. 

 Peter Sandstone in Minnesota marks the interval from lower 

 Beekmantown to upper Stones River, which interval is repre- 

 sented by several thousand feet of calcareous sediments in other 

 regions distant from the shore of that time. 



In marine transgressive overlaps, later members overlap 

 earlier ones toward the source of supply, i.e., towards the old- 

 land. In non-marine progressive overlaps, later members 

 overlap the earlier ones away from the source of supply. Thus 

 in a growing alluvial cone, the later formed beds will extend 

 farther out on to the plain away from the mountain. If several 

 successive fans of this type are formed one above the other, 

 owing to successive elevations of the source of supply, only the 

 latest beds of each delta will be found on the outer edge of this 

 compound delta, the hiatus between the beds being further 

 emphasized by the erosion which the last bed of the first delta 

 underwent during the time that the early beds of the second 

 delta were deposited nearer the source of supply, i.e., before the 

 last bed of the second delta covered up the remnant of the last 

 bed of the first delta and thus protected it from further erosion. 

 A good example of this type of overlap appears to be presented 

 by the Pocono, Mauch Chunk, and Pottsville beds of the Appa- 

 lachian region. These formations are, with exception of the 

 negligible Greenbrier member, of non-marine origin, repre- 

 senting the wash from the growing Appalachians. In western 

 Pennsylvania only the latest beds of each (barring portions 

 removed by erosion between the deposition of the successive 

 fans) are found resting one upon the other, the interval between 

 the beds becoming less and less toward the anthracite regions. 



A. W. Grabau, 



Secretary. 



SECTION OF BIOLOGY. 



May 8, 1905. 



Section met at 8.15 p.m., Vice-President Wheeler presiding. 

 1 See ante, p. 591, April meeting. 



