Geology of the Lakes and the Valley of the Mississippi. 207 



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bars of the Susqucliannah : by the combined force of a cobjnin a 

 hundred and sixty feet high — equal to the pressure of a hundred and 

 forty two inches of quicksilver, or five atmospheres — they would be 

 driven away like feathers. It is probable, therefore, that tlie bot- 

 tom of the trough is the floor of the shale, and that it will continue to 

 be so ; for the portion of the latter which lies too deep for the action 

 of frost, is sufficiently worn away by descending masses of drift-wood 

 and ice, to keep the process of excavation perpetually going on as it 

 has done heretofore. - ' - 



But taking the fact of retrocession to be established in its greatest 



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extent, yet the rate of it must have varied with tlie qualities of the 

 rock and the temperature of the seasons; so that to get at a mean 

 difference, would require a knowledge of the period already elaps- 

 ed — the very probletn to be solved. Even if there were no varia- 

 tion in these, the experience of half a century, duj'ing which the cat- 

 aract has been an object of attention, lias produced no data that can 

 lead to any certain result as to the progress of it even in that period. 

 The subject is essentially one of doubt ; and the views of Professor 

 •Rogers in respect to this part of it, present difficulties that are not 

 easily disposed of. His views, also, of the predicted catastrophe at 

 the cutting of the barrier, as it is erroneously called, seem to be well 

 founded ; nor will the subsidence of the w^ater be the less gradual 

 though the channel be carried alon^j the floor of the shale to the low- 

 est depths of the lake, 



That a wide spread current, although not, as imagined, fed from 

 an inland sea, once swept over the entire region between the Alle- 

 ghany and the Rocky Mountains, is established by plenary proof. 

 An inland sea could not have brought to their present beds, the prim- 

 itive or volcanic fragments with which the surface for more tlian a 

 thousand miles in every direction, is overspread. At the cataract 

 «f Niagara, and within a few paces of the table rock, lies a mass of 

 sienite such as occurs in the neighborhood of Philadelp!)ia ; and at 

 the same spot- are rolled pieces of common hornblende. From the 

 niuseum there, the writer of this notice obtained a fine specimen of 

 actynolite, broken, as he was assured, from a boulder in the immedi- 

 ate neighborhood. Along the whole line of coast thence to Detroit, 

 he saw spheroidal njasses of sienite w^ashed up on the beach ; and 

 o^ the shore of lake St. Clair, he found the same masses with horn- 

 blende and granular quartz. Covered with a subsequent deposit of 

 hnm wliich ^aves to the valley of the Mississipi its surpassing fertil- 



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