346 Annals of the Carnegie Museum. 



The Andropogon dune-formation of independent origin has typically 

 the following structure : 



Fades. — 



Andropogon fiircatus. 



Principal Species. — 



Arte7nisia canadensis, Euphorbia polygonifolia, 



Artemisia caiidata, Lathyrus maritimus. 



Secondary Species. — 



Asclepias syriaca, Panicum virgatum, 



Panicuin Scribnerianum. 



The general topography of an Andropogon dune, either when fol- 

 lowing Aniniophila or when originating independently, is that of a 

 low mound or ridge with gentle slopes. Succeeding the Andropogon 

 dune formation are practically the same formations as enumerated in 

 the successions starting with the Populus dune or ridge, viz., either 

 (t?) heath, (<^) white pine, and (^r) black oak ; or {a) Toxicodendron 

 thicket, (<^) wild cherry, and (<:) black oak. 



The Prunus piiniila Dune-formation. 



The sand-cherry, Prunus pumila, is a low shrub forming clumps, 

 often several yards in diameter, which are capable of stopping the 

 drifting sand and of building up a considerable dune without the 

 vitality of the plant being impaired. This species is quite common 

 about certain portions of the Great Lakes, but is not common at 

 Presque Isle. There are perhaps eight or nine small clumps of the 

 species on the interior portion of the sand-plain and about as many 

 more on the wind-swept narrow portion of the peninsula between the 

 Chimney Ponds and the Head. 



The dune formed by this species is usually small, but quite steep, and 

 is more symmetrical than dunes formed by the other dune-building 

 plants on Presque Isle. The largest Prunus pumila dune observed, 

 near the remains of the Pier of 1839, is about four feet in height and 

 eleven feet in diameter (see Plate XXXIX) . No other species than the 

 facias occurs on the Prunus pumila dunes, and the succession could 

 not be definitely determined. Probably most of the dunes disappear 

 with the death of the Pru7ius, but, possibly Prunus virginiajia or 

 Toxicodendron pubescens may sometimes act as a dune-holder and, 

 together with Rubus, etc., finally pass into a thicket-formation. 



