DETERMINATIOXS OF ALTITUDES. 9 



proved throughout its entire extent to be accurate within close approxima- 

 tion by its agreement with the railway surveys, the comparisons being 

 made at intervals \arving from 20 to 40 or 50 miles. A very large number 

 of railway profiles, extending beyond tlie limits of Lake Agassiz to Lakes 

 Superior and Michigan and to the Pacific Ocean, were examined during 

 this work, and the altitudes of their stations, summits of grade, bridges, 

 and low and high water of the streams crossed, were tabulated for con- 

 A'enient reference and comparison, being uniformly referred to the sea-level 

 at mean tide. This, auxiliary part of the investigations relating to Lake 

 Agassiz has been separately publLshed.^ In the present volume the alti- 

 tudes of the railway stations are noted on Pis. XXIII to XXXIII, which 

 give the detailed surveys of the lake beaches and deltas. For the whole 

 area of this glacial lake, so far as it has been explored with leveling and 

 is traversed by these railway surveys, their altitudes are noted on PI. X. 



Exact or close agreements of several independent surveys from the 

 sea to this district, and of the profiles of the many intersecting lines of 

 railway in Minnesota, South and North Dakota, and Manitoba, give com- 

 plete assiirance tliat these heights, and those determined therefrom by 

 leveling along a thousand miles or more of the shore-lines of Lake Agassiz, 

 are not only consistent together but also absolutely true within limits of 

 error probably nowhere exceeding 5 feet. Such exact determinations of 

 the elevations of the beaches seem very important, because these deposits 

 wdiich were formed along the level shores of the lake in its successive 

 stages are found at the present time to have a gradual ascent from south to 

 north, amounting, within the portion of the lake area surveyed by me, to 

 about a foot per mile in the highest and oldest beach, and gradually 

 diminishing to a quarter or even an eighth part of this amount in the 

 lowest and latest of the beaches. Some interesting problems are thus 

 presented as to the relationship of these progi-essive changes of level, 

 when they were produced, and their causes. 



During the year of my exploration in Manitoba, and since that time, 

 imjiortant observations of the beaches of Lake Agassiz farther noi-thward 

 along the Manitoba escarpment and near the mouth of the Saskatchewan 



• U. S. Geol. Survey, Bulletin No. 72. Altitudes between Lake Superior and the Rocky Mountains. 

 1891. Pp. 229. 



