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 
‘ general way. He 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 Rafael 
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 (6, 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 munch 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 
