Another illustration exists at the mouth of the Colorado River. As has been shown by Blake in the fifth volume of the Pacific Railroad Reports (p. 236), the delta of the Colorado—or in other words the alluvial cone which is built at its mouth—has extended itself completely across the Gulf of California, severing the upper end from the lower and from the ocean, and converting it into a lake. In continuing the upbuilding of the delta the river has flowed alternately into the lower gulf and into its severed segment. At the present day its mouth opens to the lower gulf; but at rare intervals a portion of its water runs by the channel known as “New River” to the opposite side of the delta. While it is abandoned by the river the lake basin is dry, and it is known to human history only as the Colorado Desert. Its bottom, which is lower than the surface of the ocean, is strewn with the remains of the life its waters sustained, and its beaches are patiently awaiting the cycle of change which is slowly but surely preparing to restore to them their parent waves. [Illustration: FIG. 66.—Cross-section of inclined strata, to illustrate Monoclinal Shifting of waterways. ] _Monoclinal Shifting._ In a fourth manner drainage lines are unstable. In a region of inclined strata there is a tendency on the part of streams which traverse soft beds to continue therein, and there is a tendency to eliminate drainage lines from hard beds. In Figure 66, _S_ represents a homogeneous soft bed, and _H_ and _K_, homogeneous hard beds. _A_ and _B_ are streams flowing through channels opened in the soft rock, and in the hard. As the general degradation progresses the stream at _a_ abrades both sides of its channel with equal force; but it fails to corrade them at equal rates because of the inequality of the resistance. It results that the channel does not cut its way vertically into the hard rock, but works obliquely downward without changing its relation to the two beds; so that when the degradation has reached the stage indicated by the dotted line, the stream flows at _a_, having been shifted horizontally by circumstances dependent on the dip and order of the strata. At the same time the stream at _B_, encountering homogeneous material, cuts its way vertically downward to _b_; and a continuance of the process carries it completely through the hard rock and into the soft. Once in the soft it tends like the other streams to remain there; and in the course of time it finds its way to the lower edge and establishes a channel like that at _A_. [Illustration: FIG. 67.—Ground plan of outcrops of inclined strata, to illustrate the results of Monoclinal Shifting. ]