124 MOLLUSCA. 



est lakes and swamps have an alluvial deposit partly or entirely sur- 

 rounding them. These deposits may be reconverted into swamps 

 artificially by the Ijurning of the peat that composes them, or naturally 

 by the formation of sand dams thru the agency of the wind. 



North and Stony Islands. — When the water retired from the Algon- 

 quin level, North and Stony Islands appeared as rocky islands covered 

 with a thin layer of clay or sand and the currents attacking them from 

 the north-west soon laid bare these shores. On Stony Island, the Max- 

 ville limestone outcropped for some distance above water, but on North 

 Island the outcropping along the north-west shore was mostly under 

 water. The soil from the exposed portions of these islands and more 

 sand from the shallow water off shore was carried about the ends of 

 each of them and deposited in long bars which converged toward a 

 point some distance southwest of each. A large amount of sand was 

 also deposited between the bars around each of the islands, so that 

 North Island came to have a long, submerged triangle of sand stretch- 

 ing southeast from it, while Stony Island acc[uired on the south two long 

 converging ridges of sand above the water with a large swamp between 

 them. A bar of sand along the south side of North Island makes the 

 water very shallow for a considerable distance off shore; in fact, there 

 is a long sand bar, reaching from the eastern shore of that island to a 

 sand point on the eastern shore of Stony Island, along which the water 

 is so shallow that it is grown up with bulrushes along the entire length. 

 A similar strip lies between Stony and Katechay Islands. 



There is a low ridge of alluvial soil all of the way around North Island 

 while the central portion is low and swampy. Only the northern por- 

 tion of Stony Island corresponds to North Island; the remainder is 

 built up of drifted sand, as described above. This northern alluvial 

 portion may be divided into a northern dry ridge and a southern swampy 

 strip corresponding to the similar divisions on North Island. A sand 

 beach, about two rods wide, along the southern side of the latter island, 

 together with the shallow water off shore, is all that represents the south- 

 ern two-thirds of Stony Island. 



Caseville and Pigeon River. — ^The region around Caseville is a young 

 coastal plain sloping gently towards the Bay, the soil of which is a cal- 

 ciferous, gravelly clay, with streaks of sand and even boulders in it. 

 forming low ridges in some places, and with high dunes between it and 

 the Bay. 



Thru this plain cuts the Pigeon River, a small stream which is very 

 sluggish until it gets within two miles of Saginaw Bay, where it becomes 

 swifter, but becomes sluggish again near the town where it cuts thru the 

 sand dunes to the Bay. On account of the very gentle slope of the land," 

 the river has already cut its valley about as deep as possible and is now 



