172 Professor H, D, Rogers^ on the Geology and [Feb. 8, 



them remarkably straight for great distances, others gently curving. 

 In many instances two narrow mountain crests unite at the two 

 extremities to inclose a deep trough-shaped valley ; some of these val- 

 leys, with their mountain borders, having a remarkable resemblance 

 to a long narrow canoe or skiff. One class have a synclinal structure, 

 the highest strata being in the bed or centre of the valley, and the 

 hard lower rocks forming the enclosing parallel mountain crests. 

 The other class are anticlinal in their form, a powerful erosive 

 action of waters having carved them out by cutting through the 

 crests of the original crust-waves, and scooping down to the softer 

 lower strata. In some portions of this middle belt the long narrow 

 ridges unite in wider mountain table lands of the general height of 

 the chain. 



The rocks of this long middle Appalachian belt are exclusively 

 palaeozoic strata, comprising all from the base of that system to the 

 upper coal measures inclusive. Scarcely a single igneous dyke 

 or mineral vein intrudes itself into this, or the next more western 

 division of the Appalachians. 



The north-western belt of the Appalachians is a long and com- 

 paratively narrow high table land, called the Alleghany Mountain 

 in Pennsylvania, the Sewell Mountain in Virginia, and the Cumber- 

 land Mountain in Tennessee. Almost the whole way continuously 

 from North-eastern Pennsylvania to Northern Alabama, it presents 

 a high escarped slope, facing south-eastward toward the middle 

 chain. This table land is composed of nearly horizontal strata of 

 the upper Devonian formations at its base, and of carboniferous 

 rocks in its higher parts, wide tracts of it being covered ex- 

 clusively by bituminous coal measures. Along its south-eastern 

 border it is gently undulated with parallel broad regular anticlinal 

 and synclinal flexures of the strata, the expiring waves of the middle 

 Appalachian belt, where these crests are stronger, steeper, and more 

 crowded. The plateau subsides to the north-west with the dying 

 out of these flexures, and sinks into the broad plain of the basins 

 of the Ohio and Mississippi. 



A longitudinal survey of the Appalachian chain shows much 

 undulation in its direction, and discovers ten distinct sections as 

 respects its trend. Five of these are straight, three of them con- 

 vex towards the north-west, and two of them convex towards the 

 south-east. 



C. The Rocky Mountains, or Chippewayan Chain. 



This very elevated mountain axis, or main water-shed of the 

 continent, consists generally of two, but in some districts of three, 

 principal ranges, each composed of many high mountain crests, 

 deep valleys, and elevated table lands. The great component 

 ranges are approximately parallel, but they are variously linked 

 and inosculated by transverse ridges. Their crest lines undulate 



