CHIEF PHYSIOGRAPHIC CHARACTERISTICS OP THE REGION. 371 



peaks irregularly distributed rather than arrayed in ranges and ridges. 

 The extremes of altitude are about 500 to 1,800 feet above tide level. 

 The general elevation is in the neighborhood of 1,000 feet. 



In the extreme eastern part of the crystalline district two principal 

 types of topography are represented. One is made up of a ver}^ much 

 broken or hilly region ; the other is a broad plain, with rather rough, 

 rolling surface. The first of these comprises the porphyry peaks which 

 constitute a portion of the Saint Francois mountains, a group of more or 

 less rugged hills which stretch away to the southwestward in an inter- 

 minable succession of tumuli of nearly uniform height. There is no 

 systematic arrangement discernible in these prominent surface features. 

 Although indistinctly defined ridges may be sometimes made out, the 

 rule is isolated mounds, large and small, rising irregularly behind one 

 another. The landscape presents a number of the more important ones, 

 all of nearly the same height, equally distant from one another, and sur- 

 rounded by smaller hills. The peaks stand out in marked contrast above 

 the surrounding valleys. Owing to the resistant character of the pricipal 

 massive rocks, disintegration goes on much more slowly than with the 

 sedimentaries. Hence the slopes of the porphyry hills are much steeper 

 than any of the others. Along the streams where the waters impinge 

 against the banks perpendicular walls are often formed, which rise to a 

 height of 200 or 300 feet before the surface of the mounds assumes a 

 normal slant. 



Being beyond the limits of glaciation, the inequalities of the indurated 

 rock surface have not been softened or modified by accumulations of extra- 

 limital detritus, and ice debris has in no way altered or obscured the 

 topographic expression. 



The present topographic features are the product of two cycles of 

 erosion. The evidence of the earlier of these is now nearly obliterated 

 by the vigorous action which has characterized the later. 



The leading features of the area indicate with great clearness both the 

 comparative resistance of the various rocks and the geological structure 

 of the region. The stratified rocks dip rapidly to the north, east and south, 

 away from the central granite mass. All the rocks are hard and resist- 

 ant, but, as compared with one another, marked differences are found in 

 this respect. With the vigorous erosion which has lately been renewed 

 the effects of the varying hardness of the several rock masses greatly in- 

 tensifies the contrasts of the relief. Hence the areas occupied by por- 

 phyry, granite, cherty limestone, ordinary limerock and sandstone all 

 differ in their topographic physiognomy, but the several types may be 

 readily grouped into two principal phases, of which one forms an upland 

 and the other a lowland plain. The first is the deepl}^ incised construc- 



XLIII— Bull. Geol. Soc. Am., Vol. 7, 1895. 



