338 Annals of the Carnegie Museum. 



In America such studies have been made in several locahties.®* The 

 dunes of Lake Michigan have been worked out by Cowles,"" and, in 

 many respects his descriptions are equally applicable to similar but 

 less pronounced structures at Presque Isle. 



The dunes at Presque Isle may be classified as follows : 



(a) Popidiis dunes or ridges, 



(^) Avunophila dunes, 



(^) Andropogon dunes, 



(^/) Pninus dunes, 



(^) Mixed Prumis-Smilax dunes. 



The Populus Dune-formation. 



The Cottonwood, Populus deltoides, occupies by far the most im- 

 portant position among dune-forming plants at Presque Isle, just as 

 Cowles finds it characterizing the shore of Lake Michigan at many 

 places, especially southward. 



During exceptionally heavy storms from the north or northeast the 

 surf often piles up the sand of the shore into sand-bars, which upon 

 the subsidence of the waves may remain more or less permanently 

 above ordinary water-levels (see Plate XXVI). Between such a bar 

 and the shore proper is usually a long narrow pond or lagoon, which 

 may be cut off from the lake by the drifting in of dry sand from the 

 beach, or, possibly, by the closing in of the ends by the waves. At 

 Presque Isle the subsequent history of such a lagoon depends largely 

 upon its size, its position with respect to drifting sand, and the rapidity 

 with which the general shore-line is advancing upon the lake, and so 

 leaving the lagoon inland. 



If the lagoon be towards the north or northwest portions of the 

 peninsula the greater exposure to strong winds generally results in a 

 rapid filling even of a large lagoon by drifting sand (see Plate 

 XXVII). Farther to the southeast the lagoon, being less exposed 

 to filling by wind-driven sand, may escape filling, until the general 

 shore-line has advanced lakeward to such a distance as to leave the 

 lagoon inland, where the advancing vegetation more and more prevents 



6«Hill, E. J. I.e., pp. 419-436; Kearney, T. H. "The Plant- Covering of 

 Ocracoke Island," /. <r., pp. 270-271 ; " Botanical Survey of Dismal Swamp,"/, c, 

 pp. 368-393 ; rt«f/Gleason, H. A. /. c, pp. 141-189. „ 



^^Cowle.s, H. C. "Sand-Dunes of Lake Michigan," /. r. "Physiographic ■ 



Ecology of Chicago and Vicinity," /. c. 



