INTRODUCTION 7 



Likewise differences in the amount of mean annual precipitation, which 

 range from 28-44 inches in various parts of the state, need no discus- 

 sion since rainfall in all regions is sufficient to maintain lakes at their 

 normal level. 



The underlying geological formations are, however, of prime import- 

 ance and have a marked influence on the distribution of plankton or- 

 ganisms, especially the plankton desmids. Within the state are found 

 areas of igneous, metamorphic, and sedimentary rocks each of which 

 have a different direct influence on the chemical environment of the 

 plankton organisms. The underlying rock formations of the south- 

 eastern lake area are the Niagara limestone of the Silurian; the Cin- 

 cinnati shale, Galena limestone, Lower Magnesian limestone and St. 

 Peter sandstone of the Ordovician. In the northwestern lake region 

 is found the Cambrian Potsdam sandstone, the Ordovician Lower Mag- 

 nesian limestone, together with areas of Keeweenawan trap rock and 

 Precambrian granites. The northeastern area is entirely Precambrian 

 granites. 



Overlying all of these regions is a mantle of glacial drift some of 

 which is local in origin while other constituents have been transported 

 from distant regions. This ground moraine in southeastern Wisconsin 

 varies in thickness from a few feet on hill tops to over 400 feet in the 

 bottom of preglacial valleys. Mechanical analyses show that 13% con- 

 sists of crystalline rocks transported from Canada while the remaining 

 87% is local sand- and limestone. In the northeastern lake area the 

 glacial mantle varies from 75-100 feet in thickness with a maximum 

 depth of 350 feet at the southern boundary of the area. This glacial 

 accumulation is not the result of one ice invasion but of several; the 

 last, and the one which left the most material, came from the north- 

 east and transported the conspicuous sand deposits of Vilas and north- 

 ern Oneida counties from the sandstone region of northern Michigan. 

 Previous ice invasions came from the northwest and their deposits, in 

 some instances, have not been disturbed by succeeding invasions. None 

 of the drift in this region is calcareous so that all soils are acid in 

 character. The mantle of drift in the northwestern area varies from 

 500 to a few feet in thickness and is composed of material from granitic 

 and sandstone regions. There is, however, a certain amount of lime 

 in the drift at the southern limit of the lake area. 



Chemical composition of the lake waters. Birge and Juday* have 

 made investigations on the chemical composition of the waters of Wis- 

 consin lakes in connection with their biochemical studies. They divide 

 the waters into three general classes; soft, medium, and hard. These 



* Birge, E. A. and Juday, C. The inland lakes of Wisconsin. The dissolved 

 gases of the water and their biological significance. Bull. 22 Wis. Geol. and 

 Nat. Hist. Survey. (Sci. Ser. 7.) pp. 1-259. 1911. 



