102 G. K. Gilbert—The Colorado Plateau Province. . 
independent of the displacements. Such a drainage system is 
said to be antecedent to the structure. 
There is one other case. Suppose a rock series that has been 
folded and eroded, to be again submerged, and to receive a new 
quent. How much is Super-imposed, and how much ante 
cedent remains to be determined. With the solution of the 
Gaioy of the Green and Colorado Rivers, and the physical 
history of the great Tertiary lakes; and we may hope that from 
its discussion will result the establishment of laws, by the aid 
of which it shall be possible, in other regions, to deduce facts of 
geological history from an examination of the relation of struc 
ture to drainage. 
Summary. 
The exposure of the rock structure in the Colorado Plateal 
province 1s exceptionally thorough. Soil and vegetation ob- 
struct the view less than in other lands, and deep cafions exhibit 
natural sections in many directions, 
The rock structure is simple but not the simplest. The 
strata have been displaced, but their displacement is so little 
_ The facilities for the study of single, simple displacements, 
isolated from other phenomena of the same order, are equallet 
by those for the study of eruptive mountains which are at once 
simple, isolated, and dissected by erosion. 
© the student of stratigraphy are offered continuous expo 
ures of great length, He 
To the student of erosion are exhibited the most distil 
guished monuments of its action; and he is given an opportt” 
