460 F. J. H. Merrill— Post- Glacial History 



with the theories exposed in the body of this article. The 

 stations on continents whether in Africa or America show a 

 defect of several oscillations per day in the movement of the 

 pendulum ; whereas those islands that are surrounded by a 

 deep sea and considerably removed from continental masses 

 invariably give an excess of gravity. The Barbados, in the 

 West Indies, has about a normal value, which is just what we 

 expect since it is neither surrounded by a deep sea, nor is it 

 very near the South American Continent. Observations were 

 made at St. Helena and Ascension both at the sea level and on 

 the summit of the mountain. The work shows that the mean 

 density of the whole island in both cases is considerably less 

 than that of the rocks found on the surface, so that the attrac- 

 tion of the mountain must in some way be compensated for 

 by the internal structure or composition of its material. 



Art. LIY. — On the Post- Glacial History of the Hudson 

 River Valley ; by Frederick J. H. Merrill. 



Fro^i the post-Glacial deposits in the Hudson River valley 

 may be derived much information as to the conditions prevail- 

 ing therein subsequent to the retreat from its vicinity of the 

 continental glacier. 



These deposits are of two general types : estuary formations 

 of stratified clay and fine sand deposited in still water, and 

 cross-bedded delta deposits of coarser material. They fringe 

 the river shores in terraces between New York and Albany 

 and indicate a long period of submergence, their present alti- 

 tude above tide showing that the land has been elevated with 

 respect to sea-level since their formation. Their materials 

 were apparently brought into the estuary by tributary streams 

 which dropped the coarser particles near their mouths, while 

 the finer rock flour was carried on in a state of suspension, and 

 was finally precipitated to form beds of clay. 



From Albany westward spreads an alluvial formation which 

 attains at Schenectady an altitude of about 340 feet and extends 

 through the Mohawk Yalley in terraces which rise in altitude 

 till they merge in the elevated beach plain of Lake Ontario at 

 Rome about 405 feet above tide level. The origin of these 

 terraces has not yet been determined. According to Mr. G. 

 K. Gilbert the raised beach at Rome is that of a lake dammed 

 by ice in the St. Lawrence Yalley and flowing out into the 

 Mohawk Yalley, which carries its drainage into the Hudson 

 estuary. According to this view the alluvial plain at Schenec- 



