154 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



ance of being an obsequent stream, of unusual length, it is true, but 

 associated with receding cliffs of unusual number and strength, in a 

 region of extraordinary denudation. The growth of this creek and its 

 branches, like that of the Paria headwaters, must have been at the 

 expense of many pre-existent consequent streams. The essential prin- 

 ciples of the development of such streams were stated by Powell in a 

 o-eneral way. Ho said, speaking of a series of receding cliffs, facing 

 southward : "As the cliffs are undermined, . . . the area with a south- 

 ern drainage would be increased, the area with a northern drainage cor- 

 respondingly diminished" (a, p. 210). Extensive changes of this kind 

 must have gone on during the great denudation of the plateau cycle, 

 and the growth of long obsequent streams is a natural, almost a neces- 

 sary accompaniment of the great recession of the cliffs that flank the 

 High plateaus on the south. 



The Streams of the San Rafael Swell. — Curtis creek and San Eafael 

 river were not within our field of observation, but they gain importance 

 from having been described as typical antecedent streams by Dutton, 

 who thus explained them because they run across the San Rafael swell 

 without regard to its structure (b, p. 63). They may, however, be 

 equally well regarded as superposed through overlying Tertiary strata 

 which may have once covered the denuded swell unconformably. They 

 would thus be associated with a part of Fremont river — called Dirty 

 devil river by Powell (a, p. 67), and Gilbert (a, p. 130, Plate I.), — 

 which Dutton explains as having been superposed on the mesozoic strata 

 of the Waterpocket flexure (a, p. 288), although regarding it as ante- 

 cedent in its original course on the Eocene (a, p. 282). It has already 

 been pointed out that the swell and the flexure are neighboring struc- 

 tures, involving the same series of strata. Dutton demonstrates that 

 the flexure is of pre-Tertiary date (a, p. 288. c, p. 215), and Gilbert 

 comes to the same conclusion (c, pp. 10-12) ; for Cretaceous strata 

 are involved, and their bevelled surface is unconformably covered by 

 the horizontal Eocene. It is eminently possible, as has already been 

 suggested, that the swell is of the same date, and that its truncated 

 uplift was buried by the Tertiary strata, which certainly once stretched 

 over it. Either superposition or antecedence would locate the two 

 streams on the swell without regard to its structure ; but of these two 

 processes the latter seems to me much the less probable for the reason 

 that, if the whole series of strata had been domed, and if the antecedent 

 streams had had to cut down through the successive alternations of 

 strong and weak strata from Tertiary to Carboniferous, a greater amount 



