536 CENOZOIC TIME — MAMMALIAN AGE. 



the continents, which are without stratification or order of arrange- 

 ment, and have been transported and distributed without the 

 agency of rivers. The loose, unstratified gravel and boulders over 

 New England, New York, and the States west over the same lati- 

 tudes is called drift. In some exceptional cases it is in layers; and 

 it is then called modified drift. 



Age of the Drift. — The old unstratified Drift in North America 

 never covers the river or lake alluvial terraces, either the upper or 

 lower, nor the elevated sea-beaches, and therefore preceded in 

 origin the formations of the Champlain and Terrace epochs. Sec- 

 tions have been observed in many parts of New England, Canada, 

 the region about Lake Superior, and elsewhere, confirming this 

 statement. 



General geographical distribution of the Drift. — The Drift occurs over 

 nearly all Canada, New England, and Long Island, either in view 

 or beneath the alluvium of the river-valleys. From New England 

 it stretches on westward over a broad range of country reaching 

 beyond the Mississippi, having its southern limit near the parallel 

 of 39° in southern Pennsylvania, Ohio, Indiana, Illinois, and Iowa, 

 while its northern is undetermined. South of the Ohio River it is 

 hardly traceable. Few boulders are found about Baltimore and 

 Philadelphia, and these not on the higher lands. It is thus north- 

 ern in its distribution, and, though mere gravel, sand, and stones, 

 the travelled mass has a vast extent over the northern portions of 

 the continent. It is underlaid by clayey deposits in some regions. 



Distribution in elevation. — While covering the lower country, the 

 Drift extends high up the mountains, — to an elevation of nearly 

 6000 feet on Mount Washington, 2000 or more on the Green Moun- 

 tains, 3000 on Monadnock. Boulders small or large are found on 

 the summits of most of the elevations of New England under 3000 

 feet, — as on Mount Holyoke and Mount Tom. 



Filling of river-valleys. — River-valleys in some cases were filled by 

 the Drift, so that the river, when again in action, had to reopen its 

 channel or make a new one. There is an old gorge of the Niagara 

 River, commencing at the whirlpool, which is thus filled. Tlie 

 stream afterwards opened for itself a new gorge through the solid 

 rocks. This may seem strange : but water has power only when in 

 motion ; and it will consequently leave its banks of earth uninjured 

 and wear its way through rocks, if the slope favor it. Other similar 

 cases in New England have been described by Hitchcock. 



Material of the Drift. — The material varies from fine sand and 

 gravel to coarse rounded stones or boulders, such as are strewed 

 thickly over much of New England and New York. The boulders, 



