1883.] NATURAL SCIENCES OF PHILADELPHIA. 71 



glaciers or isolated peaks and the great ice sheet of the glacial 

 epoch. While analogies might be drawn from the glacier of 

 interior Greenland or from the Antarctic ice-cap, he thought that 

 errors often arose from a too close comparison with more local 

 centres of glaciation. 



Referring to the subject of glacial motion, Professor Lewis said 

 that while there were not yet sufficient facts at hand to determine 

 its rate, its general direction and continuit}' were clearly shown 

 in the striae on elevated summits. He spoke of the importance of 

 distinguishingthese high-level stria? from those occurring in valleys, 

 remarking that erroneous conclusions had frequently been drawn 

 from an examination of maps of strite, where the relative elevation 

 of the individual stria? was not noted. While the striae upon 

 mountain summits indicate the general direction of the top of the 

 ice, and are uniform over large areas, those in valleys show merely 

 the local movement of the lower strata, and, conforming more or 

 less to the direction of the valley in which they occur, vary in 

 each localit}' and are therefore of minor importance. A s an instance 

 he described some stria? near White Haven, Luzerne Co., Pa. 

 Those in the valley of the Lehigh near the tovvn bore S. 35° E. or 

 approximately down the valley, while on the othei\hand, upon the 

 summit of Penobscot Knob, 1100 feet higher than the valley 

 (2250 feet above the sea), the sti'ia? bore S. 10° W., this being the 

 general direction of ice-flow across northeastern Pennsjdvania. 

 In all cases the striae are at right-angles to the terminal moraine, 

 and they therefore point S. E. in western Pennsylvania. He 

 gave other facts which he had observed in Pennsylvania and else- 

 where, all pointing to the continuity of action and consequent great 

 size of the glacier. He spoke of the probable analogy between 

 the Antarctic ice-cap, some 2500 miles in diameter, and the Polar 

 ice-cap of glacial times, and mentioned CroU's estimate that the 

 former is twelve miles thick at its centre. In speaking of a Polar 

 ice-cap, he did not mean to imply, however, that the ice was 

 necessaril}^ thickest on the Pole. As in Europe the mountains of 

 Scandinavia and Scotland were probable centres of glaciation, the 

 glaciers from which joined to form the great meiwle-glace , so in 

 America either Greenland, Labrador, the Hudson Bay region, or 

 elsewhere, may have been centres from which glaciers grew finally 

 to coalesce into one mass of ice, the top strata of which flowed 

 southward to the great terminal moraine. 



March 13. 

 The President, Dr. Leidy, in the chair. 

 Thirty-nine members present. 

 The death of Henry Scybert, a member, was announced. 



