THE EARTH'S FUNDAMENTAL FEATURES. 825 



sands of feet in depth. Another kind of inter-accumulation depres- 

 sion is the interval between volcanic mountains or igneous outflows. 



Besides these sources of valleys or depressions there are others 

 directly connected with movements in the earth's crust or mountain- 

 making processes. 



(1.) Geosynclinal Depressions: Or those made by downward bendings 

 of the earth's crust, producing broad basins, too shallow to be called 

 valleys, such as have existed over the surface of the continents and 

 along their borders during the progress of rock-making. Such de- 

 pressions are not now distinguishable from Intermont valleys. 



(2.) Synclinal Valleys: The synclinal depressions made in the fold- 

 ing of strata, alternating with anticlinals, few of which still exist as 

 valleys, or even as depressions, owing to the degradation which such 

 regions have undergone. 



(3.) Valleys or Depressions of Subsidence : Made by subsidences or 

 down-throws in connection with fractures. These depressions also 

 have generally been obliterated by denudation, excepting those of 

 recent origin. 



(4.) Fissure Valleys: The fissures that were left wide open after 

 a time of fracturing, instead of becoming filled with mineral material. 

 Many such have existed ; but denudation has, in general, obliterated 

 them. Running waters have made nearly all the profound fissure- 

 like channels of the world, and have not needed a fissure to initiate the 

 work, though sometimes thus aided. 



(5.) Intermont Valleys: Depressions made by upliftings on either 

 side of a region ; or the intervals between mountain chains or moun- 

 tain ranges : as that of the broad Mississippi valley, between the Ap- 

 palachian and Rocky Mountain chains ; that of Lake Champlain, be- 

 tween the Green Mountain range and the Adirondacks ; that of the 

 Connecticut valley ; and others, between the Blue Ridge and ridges of 

 the Appalachians. 



(6.) Contractional Depressions : Made by contraction from cooling. 

 The oceanic depression is such a depression, or rather a combination 

 of depressions so made. 



The depressions of the Great Lakes of North America are among the earth's larger 

 valleys. The surface of Ontario is 233i feet above the sea ; but its bottom is 600 feet 

 below its outlet, and over 360 feet below the sea; and Lake Superior, which at surface 

 is 627 feet above the sea, has a mean depth of about 1,000 feet. Such depressions can- 

 not be wholly valleys of erosion, or simple synclinal valleys. They may be geosyncli- 

 nal in origin; but seem rather to be depressions of the Intermont kind, and to exist be- 

 cause (p. 394) the region lay near the borders of the stable Archaean mass, against which 

 the continent to the south and east was pressed in the oscillations of later time. 



