APPENDIX. 313 



hard and shelly character ; and has, in the geological mutations 

 of the island been chiefly demolished and washed away. The 

 monumental mass of this period of demolition, called the Sugar 

 Loaf, is a proof that it contained, either by its shape, or other- 

 Avise, a superior power of resisting these means of ancient pros- 

 tration. Striking as it now appears, this is the simple story 

 which it tells. Its apex is probably level, or nearly so, with the 

 Fort Holmes's summit. Over the whole island, after these demo- 

 litions, the drift stratum was deposited. 



The German geognosts apply the term mushelkalk, to this spe- 

 cies of calcareous rock. It is, apparently, the magnesian lime- 

 stone of English writers. 



Ancient Water Lines. — Such marks appear on the most 

 compact parts of the cliffs, denoting the water to have stood, 

 during the ancient boundaries of the lake, at higher levels. 



Lake Action. — It is known that strong currents set into the 

 Straits of Michilimackinac, and out of it, from Lake Michigan, at 

 this point. The fishermen, who set their nets at four hundred 

 feet in the waters, often bring up, entangled in their nets, large 

 compact masses of limestone, which have been fretted into a kind 

 of lacework, by the rotatory motion of little pebbles and grains 

 of sand, kept in perpetual motion by the water at the bottom of 

 the lake. 



Organic Impressions. — There are cast up among the lake de- 

 bris of this island, casts of some species of orthocaratites, ammo- 

 nites, and madrepores, which appear to be derived from the cal- 

 careous rocks in place in the basin of Lake Huron. But the 

 rock strata of the island itself appear to be singularly destitute 

 of these remains. The only species which I have noticed, is one 

 that was thrown up from a well attempted to be dug, on the apex 

 of Fort Holmes, by the British troops, while they held possession 

 of. the island in 1813, 1814, and 1815. But this is uniformly 

 fragmentary. It has the precise appearance of the head of a tri- 

 lobite, but never reveals the whole of the lateral lobes, nor any 

 of the essential connecting parts. It is silicious. 



Gyseus Formation. — Evidences of the extension of this form- 

 ation to this vicinity were brought to my notice ; in consequence 

 of which I visited the St. Martin's Islands, which belong to the 

 Mackinac group. Masses of gypsum were found imbedded in the 



