228 J. W. GOLDTHWAIT ALGO^'QUIJS AJSD IROQUOIS BEACHES 



This distinguished visitor, I'resh from studies of the upwarped marine 

 strands of Scandinavia, applied himself at once to the task of correlating 

 the measurements of raised beaches which had been made in New Eng- 

 land, the maritime provinces, the Great Lakes region and the Northwest; 

 and this correlation he reinforced with hurried though accurate observa- 

 tions of his own in New England and southeastern Canada. In order 

 to show the upwarped form of the old ^'geoid surface" which he recon- 

 structed, De Geer used a device which Gilbert had used ten years before 

 in connection with Lake Bonneville^ — he drew curves of equal deforma- 

 tion of the ancient water-plane. These he named "isanabases," or "iso- 

 bases."* They were not limited to the Atlantic Coast district which 

 De Geer had personally examined, but were extended far into the in- 

 terior, on the basis of measurements already secured around the Great 

 Lakes by Gilbert, Spencer, Upham, and Todd, and in Labrador and 

 the Hudson Bay region by Bell and Low. In view of the scarcity of 

 data which De Geer had at his command and of the difficulties in corre- 

 lation which he met by ^^interpolating" his geoid surface beneath de- 

 formed lake beaches of unknown ages, it is no wonder that his isobases 

 for the Great Lake region hit wide of the mark. It would hardly have 

 been surprising if his conclusions had been thoroughly discredited by 

 later detailed investigations. On the contrary, however, De Geer's most 

 fundamental conclusions seem to have been confirmed. The analogy 

 which he drew between the North American and the Scandinavian up- 

 lifts appears to hold good ; in both cases the movements were differential 

 uplifts of a glaciated area, and in both the isobases follow so closely the 

 boundary of pre-Cambrian areas that a causal connection between pre- 

 Cambrian oldlands, glaciation, and differential uplift is strongly sug- 

 gested. 



Since De Geer's map was published, the use of isobases seems to have 

 been limited almost wholly to private studies; very few isobasic maps 

 have been published, and even these concern districts of comparatively 

 small extent. This has probably been due in most cases to the fact that 

 measurements have not been numerous or accurate enough to make the 

 isobases convincing, or even passably satisfactory. The fault has been 

 in the data rather than in the method. One can easily see that an iso- 

 basic map is as valuable to the investigator of epeirogenic movements as a 

 contour map is to the physiographer. If the data which we possess are too 



2 G. K. Gilbert : Contributions to the history of Lal^e Bonneville. Second Annual 

 Report of the U. S. Geological Survey, 1882, pp. 195-197, plates xlii and xliii. 



* Gerard De Geer : Quaternary changes of level in Scandinavia. Bulletin of the Geo- 

 logical Society of America, vol. 3, 1892, p. 66 ; and op. cit.. Proceedings of the Boston 

 Society of Natural History, vol. 25, 1892, p. 457. 



