academy of sciences] FIELD WORK IN COLORADO 209 



The text of the Pueblo folio adheres to the scheme adopted for the atlas series in attempting 

 to give a generally intelligible form to a condensed account of geological structure and surface 

 features; and it must be said that the scheme is in this instance applied with greater measure 

 of success — that is, of success to be measured by the capacity of a nongeologist really to under- 

 stand the text — than is usually the case, for Gilbert's descriptions are simply phrased and are 

 exceptionally free from technical terms. But, as a consequence of its untechnical simplicity, 

 some matters are omitted to which attention might have been advantageously directed. Even 

 the section entitled ''Origin of surface forms" fails of being as penetrating in its exposition as 

 it might easily have been made, in spite of Gilbert's great interest in physiography as compared 

 with structural and historical geology. For example, the district treated being constituted 

 of gently warped and moderately faulted Mesozoic strata which have been degraded to a surface 

 of low relief, monotonously plain over large areas, it naturally exhibits among occasional residual 

 inequalities many low scarps which survive here and there along fault lines, wherever strata of 

 unlike resistance are juxtaposed. Such scarps are manifestly not the direct result of faulting, 

 but the result of unequal erosion long after faulting; hence they repeatedly illustrate the im- 

 portant physiographic principles that their height gives no indication of the amount of displace- 

 ment on the fault plane, and that their aspect gives no indication of the side of downthrow; 

 and inasmuch as the distinction between these purely erosional fault scarps and true fault scarps 

 of displacement has been frequently overlooked, it would seem as if attention might have 

 properly been drawn to this lesson of the Pueblo quadrangle as well as to its other lessons, 

 for it is not to be doubted a moment that these principles were familiar to Gilbert. Hence 

 their omission suggests that the simplicity of a folio text and the avoidance of technical terms 

 may be carried so far as to be disadvantageous to the general reader, and especially so in such 

 an instance as this, in which the features shown on the map might be very properly used to make 

 clear the meaning and value of the technical terms that are allotted to them. 



On the other hand, a general statement touching the effects of degradation brings out a 

 physiographic principle which, in spite of its simplicity, is often overlooked. Here it is first 

 pointed out that the existing surface forms, which are 1,000 feet or more below the initial 

 surface of the region, "are only one phase of a changing scene"; and it is then added that, 

 as the streams have to carry away the waste of their drainage areas, they can not cut down 

 their channels indefinitely in advance of the degradation of the interstream areas, but must 

 always preserve a fall sufficient to give their current competence to do their work as trans- 

 porting agents. "Thus the slopes of all the stream beds are automatically adjusted in a har- 

 monious system, so that the wasting of the whole district is nearly uniform and its entire surface 

 is gradually reduced"; for such is the manner in which degradation acts when a very advanced 

 stage of a cycle of erosion is reached; moreover, its action in this manner is particularly sig- 

 nificant in a region that lies 1,000 miles or more from the mouth of its main river in the sea, 

 for at such a distance inland an enormous volume of rock waste may be removed from the 

 surface by fairly uniform degradation after it has been so far degraded as to merit the name 

 of peneplain and before it merits the name, plain of erosion. 



SUBSEQUENT VALLEYS 



The treatment of certain streams and valleys in the Pueblo quadrangle calls for comment, 

 as it brings up again the question of subsequent valleys to which reference has already been 

 made in the analysis of the third chapter of the Henry Mountains report on land sculpture. 

 Here, as in the case of the fault scarps above mentioned, it is difficult to understand why Gilbert 

 did not use certain facts of the map to their full value as typical examples of a class of forms 

 for which the text might provide a name as well as an explanation. The Front Range of the 

 Rocky Mountains, composed of crystalline rocks and here known as the Wet Mountains, is 

 included in the southwestern part of the quadrangle; and along the mountain base is a valley, 

 excavated in the inclined strata of the weak Fontaine formation, inclosed on the outer side by 

 a monoclinal ridge of the resistant Dakota sandstone and drained by short longitudinal branches 

 of outflowing consequent streams. Several other smaller consequent streams, which formerly 



