56 LAKES OF NORTH AMERICA. 



bonate 0.088765 part per thousand. These figures may be safely as- 

 sumed to represent the average amount of impurities carried by normal 

 streams, and consequently indicate the character of the lakes to or from 

 which they flaw. The drainage in mountainous regions, especially whei-e 

 supplied by melting snow and ice, may be purer than these figures indi- 

 cate ; while in arid regions, where efHorescent salts frequently whiten the 

 surface, the streams are more highly charged with saline matter than when 

 the rainfall is abundant. It is to be observed that material carried by 

 streams in suspension is not included in the above, considerations. 



The reader may, perhaps, conclude from the figures just given that 

 the percentage of saline matter carried in solution by ordinary streams is 

 unimportant and of but little significance in connection with the study 

 ■ of lakes. It is true that the amount of foreign matter in solution in a 

 few gallons of river water is small, but where the volume of rivers is con- 

 sidered the amount of solid substances carried by them in solution, even 

 in a single year, becomes truly startling. Knowing the volume of a 

 stream and the percentage of mineral matter it contains, one can readily 

 compute the weight of the matter it carries in solution in a definite time. 

 This computation has been made for a few American rivers. ^ 



The average flow of Croton river, New York, is 400,000,000 gallons 

 daily. In this volume of water there are 183 tons of mineral matter in 

 solution, of which 47 tons are calcium carbonate. 



The Hudson carries daily about 4,000 tons of matter in solution, of 

 Avhieh more than 1,200 tons are calcium carbonate. 



The jNIississippi carries to the Gulf of Mexico in a single year about 

 113 million tons of mineral matter in solution, of which over 50 million 

 tons are calcium carbonate. 



These estimates are only approximately correct as they depend in 

 most instances on a single analysis and on a small number of measure- 

 ments of volume. 



The invisible loads carried by rivers are not only of interest in con- 

 nection with the study of lakes, more especially of saline lakes, but open 

 a wide field of research in reference to the chemical denudation of the 

 land, the composition of ocean waters, and the source of the material, 

 more particularly of the calcium carbonate, secreted by marine plants and 

 animals. Into this In'oader domain, however, to which our subject leads, 

 we may not now enter. 



1 The data from which the facts here stated were obtained, as well as similar information 

 concerning other streams, is given in Monograph No. 11, U. S. Geoh Surv., pp. 172-175. 



1 



