HAMILTON GROUP. 43 



species. At the foot of the hills, below these shaly layers, about 

 15 feet of light-colored limestones, in beds six or eight inches 

 thick, are the lowest seen in the outcrops ; they contain but few 

 fossils, and not any of them characteristic of a certain horizon. 

 The total depth of strata composing these outcrops is from 50 

 to 60 feet. On the north side of Thunder Bay River another 

 series of rock beds is found, which I suppose to be the next in suc- 

 cession below the section just described. The marshy, level sur- 

 face on which the north half of Alpena village is situated is under- 

 laid by light-colored limestones inclosing a profusion of Stroma- 

 topora plana and many corals, as Favosites, Cyathophyllum, Cysti- 

 phyllum, partly in silicified condition. The next deeper strata, 

 exposed on the side of the first lake terrace, contain, besides the 

 aforementioned corals, silicified stems of Favosites radiciformis, 

 Cladopora Alpenensis, Michelinia insignis, Favosites radiatus, Cya- 

 thophyllum Hallii, and numerous other fossils. Among the lower 

 ledges of this horizon a bed is almost exclusively filled with dis- 

 connected shells of Cyrtina umbonata. In further descending 

 order, beds of a coarsely crystalline, drab-colored, and sandy-look- 

 ing limestone occur, which contain only a few fossils, but interlami- 

 nated with them is a seam of much lighter-colored, smooth fractur- 

 ing lime rock full of fossils, among which Chonophyllum ponde- 

 rosum and Strombodes Alpenensis are peculiar to this stratum. 

 Still below them we find compact dark gray or bluish-colored lime- 

 stones of crystalline fracture, and almost entirely composed of 

 broken shells and Crinoid stems. This rock is quarried in several 

 localities, being the only good building-stone to be had in the 

 vicinity of Alpena. The total thickness of the rock series de- 

 scribed on the north side of Thunder Bay River is about 50 feet. 

 The lowest of the beds, all of which were in nearly horizontal posi- 

 tion, are visible in the circumference of a small, abrupt hillock on 

 which Mr. Phelps's lime-kilns stand, in an uplifted position, dip- 

 ping away from the central, bubble-like protrusion at an angle of 

 25 or 30 degrees. Several such hillocks are noticed within the 

 space of half a square mile. The central mass of the hillocks 

 is a whitish, fine-grained, pure limestone, of brecciated struc- 

 ture, and of smooth, conchoidal fracture, pervaded in all directions 

 by fissures filled with calcsparand mottled by cloudy seams of light 

 green or reddish color. It contains a great abundance of fossils, 



