28 SAND DUNE REGION OF SAGINAW BAY. 



known, constitutes an area of transition betueen a northern biota 

 found mostly north of Lake Superior, and a southern one that is 

 characteristic of Indiana, Ohio and Kentucky. This subject need not 

 be discussed in detail here, but it should be pointed out that there are 

 great differences in the extent to which different northern forms push 

 southward and southern forms northward in southern Michigan, and 

 that the determining factors are the climate and the constitution and 

 habits of the organism. 



Climate. 



The climate of the sand region may be described briefly as follows: 



Precipitation: Jefferson * has shown that the "'thumb'' of Michigan 

 is characterized by a decidedly light pi-ecipitation as compared with 

 the rest of the lower peninsula. The rainfall over the greater part 

 of southern Michigan is between 30 and 35 inches. On the west coast, 

 north and south of the Saginaw-Grand Valley and in a few places in 

 the interior the precipitation is increased to 35-40 inches, and two 

 localities in the northwestern and southwestern parts have a rainfall 

 of over 40 inches, but the greater part of the lower peninsula has a 

 precipitation of 30-35 inches. There are two areas that have a rainfall 

 of less than 30 inches. One of the latter is a small area northwest of 

 Detroit, the other is the "thumb", lying between Saginaw Ba}' and 

 Lake Hm-on. 



The decrease in the precipitation in this region would itself probably 

 be too slight to affect the facies of the biota, but the effect it may be 

 expected to have is, in the sand region, added to those due to the 

 sandy soil thus increasing the general aridity of the conditions. 



Temperature. Winchell f long ago pointed out that the southern 

 peninsula of Michigan has a much more ec^uable temperatm'e than the 

 states to the south and west. Owing to the prevailing westerly winds 

 and the presence of a large body of water on either side, the winters 

 are milder, and while the summers are cooler the possibilities of early 

 and late frosts are much less than in the region to the west of the lakes. 

 This influence is felt throughout southern Michigan but is much more 

 noticeable along the coasts, particularly along the west coast. How- 

 ever, it is not much less along the east than along the west coast of the 

 state, owing to the fact that, although the prevailing winds are westerly. 



*Eighth Ann. Rept. Mich. Acad. Sci., 78-97. 



tHarpers Monthly, XLIII (1871), pp. 275-285. 



Other references on tliis subject are: Winchell, Sparks from a Geologist's Hammer, pp, 

 200-23Z; The Grnjid Traverse Region. Proc. Amer. Assn. Adv. Sci.. 1866. PP- 84-89, 1870. pp. 

 106-117: Rept. Geol. Surv. Mich.. 1871; Walling' s Atlas of Michigan. IS7S: Zeitsch der Osterrcich. 

 Gef. fur Meteor.. VII, p. 351. VIII, 40: Anv. Rept. State Hort. Soc. (Mich.), 1880; Michigan 

 (1873). McCracken. The State of Michigan (1876). Lyster, Sixth Ann. Rept. Sec. St. Board of 

 Health of Mich., pp. 167-250. 



