366 EARTH FEATURES AND THEIR MEANING . 



cessional moraines are upon the average less than 1000 feet apart, 

 and are believed to have each been formed in a single season. The 

 delta deposits which surround the esker are of thin-banded clay, 

 and as an additional uppermost band is found outside every mo- 

 raine, these bands are also believed to represent each the delta 

 deposit of a single year. In studies extending over many years, 

 Baron de Geer, with the aid of a large body of student helpers, 

 has succeeded in completing a count of moraines and clay layers, 

 and so in determining the time to be 12,000 years since the ice 

 front of the latest continental glacier lay across southern Sweden. 

 The fertility of conception and the thoroughness of execution of 

 this epoch-making investigation recommend its conclusion to the 

 scientific reader. 



READING REFERENCES FOR CHAPTER XXV 



G. K. GILBERT. Niagara Falls and their History, Nat. Geogr. Soc. 

 Mon., vol. 1, No. 7, 1895, pp. 203-236. 



F. B. TAYLOR. Origin of the Gorge of the Whirlpool Rapids at Niagara, 



Bull. Geol. Soc. Am., vol. 9, 1898, pp. 59-84. 

 A. W. GRABAU. Guide to the Geology and Paleontology of Niagara 



Falls and Vicinity, Bull. N. Y. State Mus., vol. 9, No. 45, 1901, pp. 



1-85, pis. 1-11. 

 J. W. SPENCER. The Falls of Niagara, etc. Dept. of Mines, Geol. Surv. 



Branch, Canada, 1907, pp. 490, pis. 43. 



G. K. GILBERT. Rate of Recession of Niagara Falls, etc. Bull. 306, U. S. 



Geol. Surv., 1907, pp. 31, pis. 11. 



G. DE GEER. Quaternary Sea Bottoms of Western Sweden. Paper 23, 

 Livret Guide Cong. Geol. Intern., 1910, pp. 57, pis. 3. 



