698 A. H. PURDUE 



strata of all degrees of hardness. But in the case of horizontal 

 strata undergoing base-leveling, the conditions are quite differ- 

 ent, for then the peneplain conforms to the hard stratum or 

 strata that happen to be near sea level. If such a region 

 be subsequently elevated, the streams are revived, the region 

 dissected, and the former peneplain represented by the tops of 

 the hills, which would still be capped by the hard strata that 

 were conformable with the peneplain before the region was ele- 

 vated. Now this is exactly the structural and topographic con- 

 ditions of the Boston Mountains and their outliers. But it 

 happens that these are also the structural and topographic con- 

 ditions that would prevail in a region of horizontal strata that 

 has been elevated from beneath the ocean and is undergoing 

 the process of base-leveling for the first time. So the problem 

 presents itself as to which condition prevails in the Boston 

 Mountains, and unfortunately criteria for its solution are largely 

 if not wholly wanting. 



Ordinarily, for the determination of a peneplain we look 

 to the streams. In such cases, as is well known, the streams 

 are winding, and flow in more or less steep-sided, symmetrical 

 valleys, which are themselves cut down in wider valle3's. In 

 the Boston Mountains there is no such evidence of a peneplain. 

 The streams of the region are all young, with the characteristic 

 steep-sided gorges of such streams. So far as the writer has 

 been able to observe, there is nothing in the region indicating 

 an uplift since the present streams came into existence. Their 

 valleys are relatively wide at their mouths, and gradually 

 decrease in width back to their sources, as would be expected of 

 streams cutting into a plateau of horizontal strata. The slopes 

 are undisturbed by terraces, excepting such as those men- 

 tioned above, which are due to structure. Along the southern 

 base, the oldest of the streams have reached the temporary base- 

 level of the Arkansas River, and meander somewhat, but none 

 of them to any great extent. 



It follows that evidence of a former base-leveling, if there 

 be such, must be looked for elsewhere than in the streams. A 



