340 



SCIENCE 



[N. S. Vol. LII. No. 1345 



type area, therefore, ends ■with the Choneies 

 flags of the upper Ludlow. 



These correlations have been accepted by L. 

 D. Stamp for Shropshire and South Wales.^ 

 In southern Wales the Grammysia heds are re- 

 garded as transitional between the upper Lud- 

 low and the lower Gedinnian, here the Trich- 

 rug beds. 



The evidence for drawing this boundary be- 

 tween the Silurian and Devonian systems is 

 primarily based on diastrophism, though fos- 

 sil's have always been given full consideration, 

 lithology being regarded as of least value. 



It now appears clear that the black lime- 

 stones of Bohemia known as the Ffi beds, and 

 the Tentaculite limestone or the Manlius of 

 !N"ew York must also go into the Lower De- 

 vonian. Just where the division line in Mary- 

 land, Pennsylvania, and ISTew Jersey will be 

 dra,wn is, however, not so clear, for here there 

 appears to be a more or less complete transi- 

 tion from the Silurian (Tonoloway) into the 

 Manlius equivalent. The last worker on this 

 problem, J. B. Reeside,^ was not able to adjust 

 the matter. Charles Schuchert 



THE MEASUREMENT OF POSTGLACIAL TIME 



To THE Editor of Science : The proposal of 

 DeGeer to measure postglacial time in North 

 America by the lamination of glacial clays 

 and its criticism by Fairchild are of special 

 interest to phjrtogeographers who see in early 

 postglacial migrations of plants the funda- 

 mental explanation of the present conditions of 

 plant distribution. Pairchild has taken excep- 

 tion to some of DeGeer's statements, especially 

 his estimate of 20,000 years for postglacial 

 time, and has apparently adopted Taylor's com- 

 putation of 75,000 to 150,000 years for the re- 

 cession of the ice from Cincinnati to 

 Mackinac. In this connection it is of inter- 

 est to refer to a paper of DeGeer's published 

 in 1908. In it he stated that the recession of 

 ice in southern Sweden was as slow as 25 

 meters per year, rose to 130 meters, stopped 

 for 100 to 200 years, began again at 20 

 meters, and gradually accelerated to 400 



2 Geol. Mag., April, 1920, pp. 164-171. 



3 Prof. Paper 108-K, U. S. Geol. Survey, 1917. 



meters per year. If one assumes DeGeer's 

 minimum figure of 20 meters as an average 

 annual rate in Michigan and Ohio, 36,000 

 years would be sufficient to cause an ice re- 

 cession from Cincinnati to Mackinac. Since 

 this region is farther south and with less 

 rainfall than Sweden, it is fair to presume 

 that the rate was much more rapid. Assum- 

 ing DeGeer's average figure of 200 meters per 

 year, 3,600 years would have produced the 

 same result. Neither is it necessary to in- 

 voke the precession of the equinoxes to ex- 

 plain the fifteen frontal moraines on the way. 

 DeGeer states that frontal moraines were 

 formed in Sweden during a stationary period 

 of 100 to 200 years. Such periods may have 

 resulted from cylic variations in temperature, 

 as DeGeer believes, or from similar variation 

 in precipitation. The latter ■ are of course 

 well authenticated through the researches of 

 Huntington and others. Allowing 400 years 

 for such stationary periods, the total time of 

 ice retreat over this distance is still within 

 10,000 years. 



H. A. Gleason 

 New York Botanical Garden 



explorations in the panhandle of 



TEXAS 



The third expedition to northwestern Texas 

 and Oklahoma completed its labors about the 

 first of July. This expedition found more 

 than two hundred small stone buildings in 

 groups scattered through a territory approxi- 

 mately 200 X 100 miles in extent. It appears 

 that these are not distinct Pueblo type of 

 architecture but rather mark the gradual evo- 

 lution of a nomadic buffalo-hunting tribe of 

 Indians to people who lived in stone dwellings. 

 Near the Oklahoma line the buildings are 

 small and rudimentary, and as one proceeds 

 westward they increase in size and numbers. 

 The art also develops. A preliminary paper 

 has been published setting forth the observa- 

 tions on the artifacts, irrigation ditches, picto- 

 graphs and buildings. These will be mailed 

 free of expense to any interested persons by 

 the author. 



w. k. moorehead 



Andover, Mass. 



