EXPLANATION OF GILBERT'S EXCEPTION. 141 



that such high gradients originate; and the broad crests are equally the 

 direct effects of structure. The narrow crests exemplify the law of rela- 

 tive intensities, as stated above. Water sculpture is more energetic than 

 weathering upon the precipitous edges of structural blocks; hence the 

 slopes are steep and the crests narrow. 



With the lapse of time the influence of structure gradually diminishes, 

 and the stage which base-leveling has attained exerts the greater modi- 

 fying influence upon the general law. In other words, time exerts a 

 modifying influence in proportion to its quantity, measured from the 

 beginning of the process of 1 rase-leveling. If this process has just begun, 

 structure is supreme; but if it has reached its later stages the accumu- 

 lated effects of time are supreme. Weathering and water sculpture both 

 tend to become less energetic as the surface approaches base-level and the 

 gradients flatten out, but the former retains a greater relative efficiency. 

 The transportation and removal of the solid products of weathering does 

 indeed steadily diminish, but solution — one form of weathering — and the 

 transportation of its products goes on to the very last stages of land 

 sculpture, after erosion has ceased. These last stages are therefore marked 

 by water curves of slight declivity and weather curves of great breadth 

 and flatness, both in fact closely approaching, but never quite reaching, 

 an absolute base-level. 



The general law with its modifications ma} r be summed up thus : In 

 early stages of base-leveling the predominance of water sculpture' gives 

 steep slopes and narrow crests, except where the latter have a breadth 

 which is imposed upon them by the structure, and in late stages of base- 

 leveling the predominance of weathering gives water curves of gentle 

 declivity and broad, low weather curves. 



The second observation suggested by the quotation from Gilbert is 

 that the principles explained by him as applying to "bad-land forms " 

 are equally applicable to all kinds and all stages of land sculpture. Bad 

 lands constitute a certain striking phase of land sculpture, but they are 

 nowise exceptional, so far as the general laws and processes of land sculpt 

 ture are concerned. All of the factors are present and active. The resul- 

 is unique, not because of the absence of any familiar factor nor because 

 of the presence of any new factor, but solely because of the relative 

 intensity of the factors. Structure and water sculpture strongly predomi- 

 nate over weathering. Structural forces have supplied the canon clays 

 and marls as raw material — a matrix soft, homogeneous and peculiarly 

 susceptible to rapid erosion on account of these properties and its con- 

 siderable elevation. Water sculpture, attacking it with an energy pro- 

 portional to its height above base-level and its lack of cohesion, cuts deep 

 gashes so rapidly that weathering has little opportunity to round off the 



XXI— Bull. Geol. Soc. Am., Vol, i, 1882. 



