. Conservation. Forests and forestry. 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 com
. Conservation. Forests and forestry. 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
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