NARRATIVE OF THE EXPEDITION. 61 



in the slate of calcspar, crystallized in minute crystals. The stra- 

 tum on which this loose formation rests is compact and firm, 

 and agrees in structure with the encrinal limestone of Drummond 

 Island and the Manitouline chain. But the vesicular stratum, 

 which may be one hundred and ten or tAventy feet thick, has been 

 deposited in such a condition tliat it has not had, in some local- 

 ities, firmness enough permanentl}^ to sustain itself. The conse- 

 quence is, that the table-land has caved in, and exhibits singular 

 depressions, or grass-covered, cup-shaped cavities, which have no 

 visible outlet for the rain-water that falls in them, unless it per- 

 colates through the shelly strata. Portions of it, subject to this 

 structure, have been pressed off, during changing seasons, by 

 frosts, and carried away by rains, creating that castellated appear- 

 ance of pinnacles, which gives so much peculiarity to the rocky 

 outlines of the island. 



The ARCHED ROCK is an isolated mass of self-sustaining rock, 

 on the eastern facade of cliffs ; it offers one of those coincidences of 

 geological degradation in which the firmer texture of the silicious 

 and calcareous portions of it have, thus far, resisted decomposi- 

 tion. Its explanation, is, however, simple : The apex of this 

 geological monument is on a level, or nearly so, with the Fort 

 Holmes summit. While the diluvial action, of Avhich the whole 

 island gives striking proofs, carried away the rest of the reticu- 

 lated or magnesian limestone, this singular point, having a firmer 

 texture, resisted its power, and remains to tell the visitor who 

 gazes at it, that waters have once held dominion over the highest 

 part of the island. 



Before dismissing the subject of the geological phenomena of 

 this island, it may be observed that it is covered Avith the erratic 

 block or drift stratum. Primitive or crystalline pebbles and 

 boulders are found, but not plentifull}^, on the surface. They are 

 observed, however, on the highest summit, and upon the lower 

 plain ; one of the best localities of these boulders, exists on the 

 depressed ground, leading north, in the approach to Dousman's 

 Farm, where there is a remarkable accumulation of blocks of 

 granite and hornblende drift boulders. The principal drift of the 

 island consists of smooth, small, calcareous pebbles, and, at deeper 

 positions, angular fragments of limestone. Sandstone boulders 

 are not rare. Over the plain leading from the fort north by 



