134 MOLLUSCA. 



up islands all over the lake, varying in size from a few feet to several 

 hundred yards across. The larger ones are covered with spruce, tam- 

 arack and a, few white pine, with an undergrowth of sphagnum, pitcher 

 plant, sundew, huckleberries and poison sumach. As the water in most 

 places is not over a foot or two in depth, and nowhere is over four or 

 five feet deep, while the peat deposit is in places over twenty feet deep 

 and the tamarack belt around the lake attains a width of oyer a mile, 

 Rush Lake is apparently in a late stage of its life history. 



1 1 . Mucky bottom . The bottom over the greater portion of this lake, 

 as mentioned in the general discussion, is finely divided, peaty muck, 

 so soft that it is almost impossible to distinguish, by feeling, the upper- 

 most layers from the water over them. This layer of muck varies from 

 a few inches to over twenty feet in depth, and is underlaid thruout the 

 northern, western and most of the eastern portions with sand or gravel, 

 and thruout most of the southern side by clay and, in a few places, with 

 Marshall Sandstone. The water over most of the lake is very shallow 

 but along the northern side there is a deeper channel, the bottom of 

 which is also mucky. Beds of bur- weed (Sparganium eurycarpum), 

 pickerel weed {Pontederia cordata), bulrush (Scirpus validus), wild 

 celery {VaUsneria spiralis), Potamogcton natans, Najas, and other water 

 plants are forming islands, by the deposition of peat, all over the shal- 

 lower portions of the lake, while the white and yellow water-lilies 

 {Castalia odorata and Nymphaea advena) and Carex filiformis are filling 

 in along the mucky shores. Also, along the deeper channel, there is an 

 abundance of vegetation, consisting mainly of several species of Pot- 

 amogeton. 



On lily-pads, Lymnaea stagnaUs appressa, Planorhis trivolvis, Physa 

 heterostrapha, Ancylus parallelus, and Armiicola limosa were collected in 

 considerable numbers. The first two species were also obtained from 

 the roots of sedges, and on the bottom in shallow places. In lesser 

 numbers, all of these species were present in the pond-weed zone, and, 

 in addition, numerous specimens of Anodonta marginata and A. grandis 

 gigantea, and a few individuals of Lampsilis luteola, were found partially 

 buried in the soft muck, especially in places where there was little vege- 

 tation. 



12. Floating sedge-marshes. Out from the shore of the lake, in al- 

 most all places where the bottom is mucky off shore, there runs out a 

 belt of floating marsh, which in many places is twenty yards wide. The 

 marsh is principally built up of Carex filiformis; there is, however, a nar- 

 row zone of a heavier sedge along the drier portions in-shore. Among 

 the sedges and helping to form the marsh are large numbers of plants of 

 the marsh fern (Aspidium thelypteris) , the common blue flag (Iris ver- 

 sicolor), arrowhead (Sagittaria latifolia), Asclepias incarnata, alder 



