312 APPENDIX. 



persecl over the continent. The fisTiermen represent tlic water 

 around this island to be eighty fathoms in depth. Yet, across 

 these waters, to the utmost altitude of the island, these blocks of 

 foreign rock have been transported. No force capable of effect- 

 ing this is now known. And the argument of their having been 

 transported on cakes of ice, in the nascent periods of the globe, is 

 rendered stronger by these appearances than any geological proofs 

 which I have yet seen. 



Distinctive Character of the Mackinac Limestone. — No- 

 thing appears so completely to puzzle the observer as the first 

 glance at this rock. It is different in appearance from the calca- 

 reous rocks, to which my attention has heretofore been called in 

 Western New York, and in Missouri and Illinois. The difficulty 

 is to find a point of comparison. I walked entirely around the 

 island, partly in water, the northern shores being comparatively 

 low. There appeared to be three layers. The first, which rises up 

 from the depths of the lake, scarcely, if at all, reaches the water 

 level. Upon this is superimposed a vesicular rock, of which the 

 vesicles are filled with carbonate of lime in the state of agaric 

 mineral. By exposure to the air, this substance readily decom- 

 poses, and assumes an almost limey whiteness, and sometimes a 

 complete pulverulent state. The reticular, or vesicular lines, by 

 which the mass is held together, are thus weakened, and large 

 masses of the craggy parts fall, and assume the condition of 

 debris at the water's edge. Some conditions of the reticulated 

 filaments are covered with minute crystals of cal. spar; others of 

 minutely crystallized quartz. There appear, at other localities, 

 in low positions, layers of quartz in the condition of a coarse 

 bluish, flint}^, striped agate. The entire stratum appears to be a 

 reproduced mass, which is plainly denoted, if I mistake not, by 

 some imbedded masses of an elder lime-rock. The whole stratum 

 is too shelly and fissured to be of value for economical purposes. 

 It yields neither quicklime nor building stone. 



Fort Mackinac is erected on the summit of this stratum. The 

 two objects of curiosity, called the Arched Rock, and the point 

 called Robinson's Folly, are evidences of this tendency of the 

 clifts to disintegration. The superior stratum which constitutes 

 the nucleus of the Fort Holmes' summit, contains more silex, dif- 

 fused throughout its structure. It is, however, of a loose, though 



