HAMILTON GROUP. 



47 



its efflux was retarded only, not prevented. At the time of my 

 visit in June I found the lake 25 feet below high-water mark. 

 At its west end a vertical rock wall about 15 feet high ascends 

 from the water; the ledges are dolomitic limestone from 18 

 inches to 2 feet thick, of light gray color, separated by thin, 

 intermediate shale seams ; they contain Atrypa reticularis, Spirifer 

 granuliferous, Strophodonta demissa, and Crinoid stems. The posi- 

 tion of the beds is visibly inclined toward the lake. The beds 

 next to these, in the ascending line, are exposed at the south 

 shore of the lake ; they contain an abundance of Stromatopora 

 Wortheni and Stromatopora monticulifera, Cyathophyllum profun- 

 dum, various species of Favosites, Alveolites Goldfussii, heads of 

 Dolatocrinus and Megistocrinus, Atrypa reticularis, Strophodonta 

 demissa, Spirifer mucronatus, Spirifer granuliferous, and other 

 fossils. The outlet of the lake when its basin is full is at the south 

 end. When I saw it, the bed of the creek lay perfectly dry, and be- 

 fore reaching its level the water would have had to rise at least 

 20 feet. 



The outlet of Long Lake, in Township 32, R. 8, east, is formed 

 by a similar large sink-hole. During the wet season this basin is full, 

 and a large creek flows in rapids and small cascades toward Lake 

 Huron. In summer-tirne, the basin, all formed of solid rock, 

 lies perfectly dry, and the water, discharged from the main body of 

 the lake above, runs off through subterranean crevices. 



The surface rock of the shore-belt from Long Lake southward to 

 the north point of Thunder Bay, inclusive of Thunder Bay Island 

 and Sugar Island, belongs all to about the same geological horizon. 

 It is composed of an alternation of limestones, partly of gray, partly 

 of black color, with interstratified seams of shales, which are also 

 black or dark gray. Some beds are of nodular, concretionary struc- 

 ture, almost entirely composed of various species of Stromatopora, 

 Cyathophyllum profundum, Favosites Hamiltonensis, Favosites 

 digitatus, and other corals and shells, particularly Brachiopods. 

 The exuberance of fossils in the strata is most beautifully exhibited 

 in the shoals of the lake north of Thunder Bay Island, where any 

 one sailing over them can see for miles the whole bottom paved with 

 corals in convex lumps, from a few inches to some feet in diameter, 

 their white, sparry substance contrasting beautifully with the dark 

 limestone which incloses them. 



