CUNCIA'SIONS 11.") 



The subject is, liowever, too extensive and too va«j;ue to ))e discussed 

 properly here, aiul hiis been introduced only to indicate the complexi- 

 ties of the problem j)resented by the ^reat series ot" raised beaches. 



The theory of ice-dams as causing the old water-levels seems {)refer- 

 able to the older one still held by some geologists, that the beaches were 

 all formed at sealevel, and has been ado{)ted in this paper, since it is 

 scarcely conceivable tliat marine fossils should swarm as they do in de- 

 ])osits U}> to the level of 350 or 440 feet and suddenly cease above that 

 level. 



Those who consider the beaches marine may reply, of course, that the 

 seashells which once existed in the higher beaches have all been leached 

 out by percolating waters, or that shells were never deposited in them, 

 owing to the difference in conditions, such as the coldness or brackishness 

 of the waters in contact with the edge of the ice. 



While it is true that the higher beaches are, in general, older than the 

 lower deposits which have abundant shells, and so may have suffered 

 more from weathering, it must not be forgotten that the comparatively 

 ancient Algon([uin sands near Georgian bay are often crowded with 

 freshwater shells still in perfect preservation, though somewhat fragile. 

 On the other hand, the supposition that the seawater was lifeless when 

 the U[)per beaches were made seems hard to defend. The coldness of 

 the water does not affect the matter, for arctic seas swarm with life almost 

 as much as temperate or tropical ones; nor can one assume that the water 

 of a sea that must have been connected by broad and deep channels 

 with both Hudson bay and the Atlantic could be made so brackish as 

 not to support marine life, when tlie later comparatively narrow and 

 shallow inlet west of Montreal left beds filled with shells, in spite of the 

 pouring in of fresh water from the upper lakes. 



The objection sometimes made to the theory of ice-dams that no gla- 

 cial mass could withstand the pressure of a head of water hundreds of 

 feet in dei)th, and that the lakes would soon find an outlet beneath the 

 ice, does not seem well taken, for no one knows how effective a dam a 

 sheet of ice 100 miles broad and a thousand or several thousand feet thick 

 would make. The modern glaciers whose ice-dammed lakes have been 

 studied are too insignificant relatively to make a comparison of much 

 value. As to the head of water to be held up, one may just as fairl}^ assume 

 a low one as a high one, for it is admitted that the whole region stood 

 lower at the time the beaches were formed tlian now, and the depression 

 may have been great enough to relieve the dam of much of the pressure. 

 It is quite unnecessary, therefore, to assume an ice-dam holding up a 

 huge lake 1,400 feet above sealevel, so as to form a beach now at that 



XXI— Bull. Gkol. Soc. Am., Vol. 12, 1900 



