i^OllTHEASTWAED OUTLETS. 227 



the earliest outlet in that direction was occupied during the time of the 

 three Blanchard beaches and the Hillsboro beach, the channel being cut 

 down about 45 feet. The comjjaratively large interval above the Emerado 

 beach may be supposed to imply the transfer of the discharge to a new 

 outlet; and the series of smaller intervals separating the next five beaches 

 may indicate that they all were formed while this channel was being cut 

 down about 100 feet. Another large fall of the lake, to the Niverville 

 beach, which is compound in its northern part, would again mark the occu- 

 pation of a new outlet. This, however, was soon abandoned for the still 

 lower course of the Nelson. Exact heights of these old river courses, 

 crossing present liufes of watershed, and the depths of their erosion, will 

 doubtless be determined at some future time by exploration and leveling, 

 though probably not until after many years, on account of the difficult}' of 

 carrying instrumental surveys through that wooded and uninhabited region. 



DEPENDENCE OF LAKE LEVELS ON BPEIROGENIC ELEVATION.' 



The five or six distinct beaches that were formed by the southern part 

 of Lake Agassiz during its outflow southward are represented in the north- 

 ern part of its basin by seventeen separate shore-lines, which are marked 

 by definite beach ridges. The individual beaches at the south, being 

 traced northward, become double or triple, and the highest or Herman 

 beach expands into seven successive shore-lines. During the earlier years 

 of my exploration of this glacial lake I believed that this duplication and 

 multiplication of the beaches observed in advancing from south to noi'th 

 was referable to the diminution of the attraction of the ice-sheet as its final 

 melting progressed. Gravitation of the lake toward the vast mass of the 

 ice, and its decrease with the glacial recession, I then su^jposed to be ade- 

 quate to explain the observed northward ascent of the beaches, amounting 

 for the highest Herman beach to 5 or 6 inches per mile for its first 50 miles 

 at the south, but thence increasing northward to 1 foot and 1^ feet per 

 mile; for the succeeding beaches, of gradually diminishing amount; and for 

 the McCauleyville beach, the latest formed during the southward outflow, 



' For the deBnition of this term, proposed by Gilbert, see page-103. 



