156 Rhodora [Juny 
In the accounts above quoted of the extension to Massachusetts 
of the Coastal Plain plants no direct mention is made of one very 
important item. This is the amount of water which would necessarily 
have been withdrawn from the ocean in order to form the tremendous 
masses of ice which, as is now becoming apparent, extended simul- 
taneously over large areas of both the northern and the southern 
hemispheres. Professor Reginald A. Daly, to whom I am indebted 
for references to papers on this subject, says: “At the time of their 
maximum extension the ice-sheets, which have since melted away, 
covered a total area which may be estimated as from five to eight 
millions of square miles." ! And though there is great diversity in the 
estimates of the average thickness of the ice and consequently of the 
corresponding decrease in the depth of the ocean with the resultant 
lowering of sea-level all over the globe, some idea of the magnitude 
of this change can be gained by the following extracts from students 
of the subject. 
Penck, in 1894, wrote: “The causes of the general rise of sea-level 
in the latest geological time might perhaps be connected with those 
climatic changes which the earth underwent in the Glacial period. 
If, during that time, northern Europe, northern North America, 
and the Antarctic regions were simultaneously glaciated, a consider- 
able mass of water must have been removed from the ocean, and, if 
the thickness of ice be assumed as 1,000 meters, the sea-level must 
have been 150 meters below its present position.” ? 
Daly, in 1910, felt that these former estimates were too great and 
that " the formation of the ice-sheets (which have since disappeared) 
would produce a negative movement of sea-level in low latitudes to 
an amount ranging between twenty-five and forty-five fathoms.” ? 
Besides the direct removal from the ocean of water sufficient to 
form the ice, there is a second factor which must be taken into account. 
This is the gravitative power of the great masses of ice. Using the 
formulas of R. S. Woodward,* Daly calculates that “if the ice had an 
area of 6,000,000 square miles and an average thickness varying from 
! Daly: Pleistocene Glaciation and the Coral Reef Problem. Am. Jour. Sci. 
ser. 4, xxx. 299 (1910). 
? Penck, Morphologie der Erdoberflaeche, ii. 660 (1894) as translated by Daly, l. c. 
298. 
3 Daly, 1. c. 300. 
* Woodward: On the Form and Position of the Sea Level. U.S. Geol. Surv. Bull. 
48 (1888). 
