APPENDIX. 805 



ancient — volcanic areas of action. These lakes form — except 

 Erie and Ontario — the general boundaries between the primitive 

 and secondary strata. But, however striking this fact may, at 

 particular localities, appear — such as at the Straits of St. Mary, 

 of which the east and west shores are, geologically, of different 

 construction — yet nothing in the grand phenomena of the whole 

 region visited is so remarkable as the boulder stratum, which is 

 spread, generally, from the north to the south. Some of the 

 blocks of rock are enormous, and would seem to defy any known 

 cause of removal from their parent beds ; others are smaller, and 

 have had their angles removed, and far the greater number of 

 these transported boulders are quite smooth and rounded by the 

 force of attrition. This drift stratum has been tossed and scat- 

 tered from its northern latitudes over the surface of the limestones 

 and sandstones of the south. It is mixed with the diluvial soils, 

 in Michigan and elsewhere; but it is evident that, in its diffusion 

 south, the heavier pieces have settled first, while comparatively 

 minute boulders have been carried over or dropped in the plains 

 and prairies of Ohio, Illinois, and more southerly regions. Nobody, 

 with an eye to geology, can mistake the heavy boulder deposits 

 which mark the southern shores of Huron, and become still more 

 abundant on the St. Mary's, the shores of Lake Superior, and 

 along the channels of the Eiver St. Louis and the Upper JMissis- 

 sippi. 



Lake Superior has been the central theatre of volcanic up- 

 heavals ; but they must have operated at very remote periods, 

 for there is not only no evidence of existing volcanic fires, but the 

 heavy debris everywhere bespeaks long intervals of quietude, and 

 slow elementary degradation. Some of the upheavals were made 

 after the deposition of the sandstone rocks, which are, as at the 

 foot of the Porcupine Mountains, raised up to stand nearly ver- 

 tical ; while other districts of the granitic rock, as at Granite 

 Point, had been elevated before the deposition of the sandstone 

 rock, which is accurately adjusted to its asperities, and remains 

 quite horizontal. 



The granitical series of strata, which is apparent in northern 



New York in the Kayaderasseras Mountains, and at the Thousand 



Islands of the St. Lawrence, reappear on the north shores of 



Iluron and Superior, underlie the bed of the latter, and rise up 



20 



