NEWS AND NOTES 



571 



change fifty per cent, in their mineral 

 content during the year, this annual 

 fluctuation 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. JMary's River to twenty-seven per 

 cent, in St. Lawrence River at the foot 

 of Lake Ontario. The chemical com- 

 position of the water does not, there- 

 fore, bear a fixed relation to the quan- 

 tity of water discharged. iMr. Dole 

 gives as the probable reasons for this 

 comparative steadiness in concentration 

 the absence oi' large tributaries and the 

 low ratio between the areas of the 

 drainage basins and the lake surfaces. 

 Though the lake waters do not 

 change greatly from month to month, 

 they dififer a great deal from one an- 

 other in concentration. Lake Superior 

 is least strongly mineralized ; Lake 

 Michigan is twice as high in total sol- 

 ids, and Lake Huron is but little less 

 mmeralized than Lake Michigan. Lakes 

 Erie 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 

 afiluents bearing large quantities of dis- 

 solved matter ; Lake Michigan and 

 Lake Huron, on the other hand, re- 



ceive drainage from limestones and 

 sandstones of the sedimentary series 

 and contain much greater proportions 

 of mineral matter. Dilution by the 

 softer water of Lake Superior probably 

 accounts for the fact that Lake Huron 

 water, is less mineralized than that of 

 Lake Michigan. It is probable that 

 forestation, sedimentation, and relation 

 of rainfall to run-off also affect the rel- 

 ative composition of the lake waters. 



Comparison of the analyses of the 

 lake waters with those of tributaries to 

 the system shows, according to Mr. 

 Dole, that the lakes are almost inva- 

 riably softer than their affluents. The 

 reason ibr this dift'erence is apparent : 

 As the lake surfaces are large in pro- 

 portion to their corresponding land 

 drainages a great part of the rain falls 

 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 leads 

 to the conclusion that the lake waters 

 may be specially recommended for in- 

 dustrial and domestic uses wherever 

 they can be economically obtained. 

 They are low in mineral content and 

 normally free from turbidity, and the 

 nature of the dissolved constituents is 

 such that they can be used for boilers 

 and for most other industrial purposes 

 without purification. 



