66 MICHIGAN SURVEY, 1908. 



as Formica dryas Wheeler, suggesting a possible confusion of the num- 

 bers. No other insects were observed except the omnipresent black-flies 

 and mosquitoes. 



At the head of the numerous fjord-like inlets along the shore there 

 is usually a swamp tract extending for some distance inland in the 

 same direction as the inlet itself. The level is but little above the lake 

 itself, but there is no permanent standing water or lakes as in the 

 tamarack swamps. The standing water is limited to small scattered 

 pools, seldom more than a meter across, and the forest cover is pre- 

 vailingly of arbor vitae. The shade is exceedingly dense, and the 

 ground " is covered with tangles of underbrush and fallen logs. The 

 fauna is accordingly reduced to a minimum, and the few forms col- 

 lected were all dredged from the leaf-covered bottoms of the small 

 pools, and included bivalve shells, PyramidiiU striatella (Anth.), and 

 Pisidkim stihrotvrulum Sterk. (No. 182), and water beetles, Scutopterus 

 homii Or. (Nq. 182). The latter were very scarce. 



Faunistically the arbor vitae swamp is very closely related to the 

 later stages of the tamarack swamp, as a comparison of the species will 

 show. At the ends and around the sides the swamp grades imper- 

 ceptibly into the balsam-spruce forest. 



In connection with the swamps must be mentioned the fauna of the 

 small rapidly flowing streams leading out of the inland lakes. The 

 bottom is usually rock or gravel, and the swift current prevents the 

 accumulation of organic debris. In Benson Brook on the north side 

 of Rock Harbor in still, deeply shaded places were dredged up (No. 149) 

 Pallifera dorsalis (Binn.), Pyrarnidula altern-ata (Say), Pyramididn 

 striatella (Anth.), Zonitoides exiginis (Stimp.) and Physa sp. In the 

 outlet from Siskowit Lake, in small pools 5-15 cm. deep with a bottom 

 of slime covered with loose pebbles, were collected several shells (No. 

 238), Physa sp., PisMinm mediamim Sterki, P- siibrotundum Sterki, 

 and Miisculin.m securis (Prime). The current where these were col- 

 lected was very slow. In the more swiftly flowing water nothing could 

 be found. 



Owing to the peculiar geological structure of the island the swamps 

 have a generally oblong form with approximately parallel sides. Along 

 the sides the swamps grade imperceptibly into the balsam-spruce forest 

 {Fig. 43), and on the ends as well, though there the transition 

 is more gradual and the facies are usually separated by an intermediate 

 zone marked by dense thickets of alder. 



V. The Gravel and Sand Beaches. 



The gravel beaches are found in but certain places along the shore 

 {Fig. 1), where the slope of the banks and the action of the waves permit 

 the formation of the , gravel deposits. Optimal conditions are found 

 at the heads of the numerous inlets or coves ; such as Conglomerate Bay 

 {Fig. 4), and Tonkin Bay, already described, and many other similar 

 places. They also occur, however, along the shore of the lake itself, 

 where the wave action is at its minimum. Their distribution appears 

 to be controlled principally by the slope of the bottom, since the gravel 

 could not be piled up on slopes of too steep pitch, and they are almost 



