\K\VS AND NOTES 



571 



change tiltv per cent, in their mineral 

 content during the year, this annual 

 lluctuation of fifteen per cent, is very 

 small. The average monthly fluctua- 

 tion in the discharge of the Great Lakes 

 is considerably more than fiiteen per 

 cent., ranging from forty per cent, in 

 St. Mary's Kiver to twenty-seven per 

 cent, in St. Lawrence Kiver at the foot 

 of Lake ( )ntario. The chemical com- 

 position of the water does not, there- 

 lore, hear a fixed relation to the quan- 

 tity of water discharged. Air. Dole 

 gives as the probable reasons for this 

 comparative steadiness in concentration 

 the absence O1 lar^e tributaries and the 

 low ratio between the area- of the 

 drainage basin- and the lake surfaces. 

 Though the lake waters do not 

 change greatly from month to month, 

 they differ a great deal from one an- 

 other in concentration. Lake Superior 

 is least stronglv mineralized ; Lake 

 Michigan is twice as high in total sol- 

 ids, and Lake Huron is but little less 

 mineralized than Lake Michigan. Lakes 

 F.rie and Ontario are practically alike 

 in mineral content, holding about two 

 and one-half times as much solids in 

 solution as Lake Superior. Reason for 

 the striking difference in the lake- 

 waters is found in the character of the 

 geologic formations in the drainage 

 basins tributarv to them. The crystal- 

 line and igneous rocks granite, schist, 

 gneiss, and basalt that predominate all 

 around Lake Superior, are not easily 

 soluble, and hence the lake receives few 

 affluents bearing large quantities of dis- 

 solved matter; Lake Michigan and 

 Lake Huron, on the other hand, re- 



ceive drainage from limestones and 

 sandstones ot the- sedimentary series 

 and contain much greater proportion- 

 of mineral matter. Dilution by the 

 sotter water of Lake Superior probahh 

 accounts f,, r the fact that Lake Huron 

 water is less mineralized than that o 

 Lake Michigan. It is probable that 

 lorestatioii, sedimentation, and relation 

 <>f rainfall to run-off al-o affect the rel- 

 ative composition of the lake waters. 



Comparison of the analyses of the 

 lake waters with tho-e of tributaries to 

 the system -hows, according to Mr. 

 Dole, that the lakes are almost inva- 

 riably softer than their affluents. The 

 reason .nr this difference i- apparent: 

 As the lake surfaces are large in pro- 

 portion to their corresponding land 

 drainages a great part of the rain fall- 

 directly into the lake waters and dilutes 

 them; on the other hand, rain falling 

 upon the land becomes more or less 

 impregnated with mineral salts before 

 it reaches the lakes in the normal run- 

 off. This fact has an important relation 

 to the industrial consumption of the 

 waters, and shows the importance of 

 locating intakes outside of the influence 

 of tributary streams. 



A study of all the data at hand lead- 

 to the conclusion that the lake water- 

 may be specially recommended for in- 

 dustrial and domestic uses wherever 

 they can be economicallv obtained. 

 They are low in mineral content and 

 normally "ree from turhiditv. and the 

 nature of the dissolved constituents i< 

 such that they can be u-ed for b<>iler- 

 and for most other industrial pur]* 

 without purification. 



