The story of our continent : a reader in the geography and geology of North America : for the use of schools . the stream. All flowing watertends to cut away the rocks over which it passes. Intime, any river will wear down all the obstructions ofits bed, so that it will flow with a gentle, continuous,though diminishing, current from its source to the sea. When a mountain range is formed, the streams im-mediately set to work to bear it away, removing therocks bit by bit to the sea. This work they also do,though more slowly, with the lower-lying rocks of thecontinental plains. Where the region h
The story of our continent : a reader in the geography and geology of North America : for the use of schools . the stream. All flowing watertends to cut away the rocks over which it passes. Intime, any river will wear down all the obstructions ofits bed, so that it will flow with a gentle, continuous,though diminishing, current from its source to the sea. When a mountain range is formed, the streams im-mediately set to work to bear it away, removing therocks bit by bit to the sea. This work they also do,though more slowly, with the lower-lying rocks of thecontinental plains. Where the region has recently beenelevated — recently in a geologic sense, though it mayhave been millions of years — above the sea, the streamsusually occupy deep gorges and river-beds, and have arapid fall. Gradually the main rivers and the tributarystreams cut down their beds, and, swinging to and fro,carry away the highlands on either side:. all the whilethe underground water is dissolving out the substanceof the rocks which are not touched by the streams ; thusin time, if the country does not continue to rise, the. OF NORTH AMERICA. 137 surface is brought down to near the level of the are formed wherever a portion of the rockover which the stream flows is peculiarly resisting tothis down-cutting action. The circumstances whichmay bring about the formation of such falls are numer-ous and interesting. The most common cause of thesesudden interruptions in the uniform flow of the streamis well represented in the Falls of Niagara. There thegreat river which drains the upper lakes, Erie, Huron,Michigan, and Superior, flows over bedded limestonesand shales which dip up the stream. The hard, com-pact layers of the Niagara limestone are but little wornby the clear water which courses over them. They giveway only as the soft shales below them are cut out bythe whirling waters at the base of the fall; the lime-stone blocks left without support then tumble in ruinsinto the gorge. In
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