GEOLOGY OF THE NORTHERN ADIRONDACK REGION 291 



into prominence as topographic features; reelevating anew the 

 eastern region as contrasted with the Chainplain valley, and re- 

 producing the comparatively rapid, steplike drop from one to the 

 other; and breaking up the old, comparatively even erosion sur- 

 face into a jumble of disconnected blocks at various altitudes. 

 Hence the present ridge and hilltops appear at all sorts of dis- 

 cordant elevations, instead of exhibiting the concordance in alti- 

 tude which is such a characteristic feature on the south and west, 

 where there was little or no faulting. 



Not far to the west of the main axis of uplift lies a central 

 depressed belt whose ridge summits fall far short of attaining 

 the elevations along that axis, and much short of attaining those 

 to the west. These differences are most accentuated through 

 Franklin county, and the belt seems to have originated as a de- 

 pressed, or dropped fault block, between the eastern and western 

 uplifted areas. Whether it originated at this time, or dates back 

 to a previous period of faulting, with renewal of its previous 

 features at this date, can not be told. 



The region remained at the new altitude given by the uplift 

 for a sufficiently long time (the greater part of the Cenozoic) to 

 permit of erosion giving it approximately its present relief. 

 Stream valleys were cut down to the new base level and on the 

 average sufficiently widened, so that one half of the region (at 

 a rough approximation) was cut down well toward that level, 

 the remainder forming interstream ridges and hills whose sum- 

 mits have been lowered little below the altitude given them by 

 the uplift. The streams had become adjusted to the rock struc- 

 ture of the region during the previous cycles of erosion, so that 

 they coincided with, and their attack was mainly felt on, the 

 weaker belts. In the heart of the region, where Precambric 

 rocks are at the surface, the weaker rocks are the Grenville lime- 

 stones and associated sedimentary gneisses. Wherever these 

 rocks occurred in belts of any extent, they would locate the line 

 of a stream valley, whose width would be rudely proportional 

 to the breadth of the belt. The remaining common rocks of the 

 region are much more resistant and with no great variation among 

 themselves in this respect, so that they present little compara- 



