CONDITIONS OF POST-TERTIARY EROSION 337 



which reached the ocean indirectly through the bay of Nicaragua. The 

 former group, of which the stream occupying the lower portion of the 

 present Rio Grande valley is the best studied example, cut their valleys 

 Avell down toward the new baselevel nearly up to the divide, while the 

 inner portion of the peneplain occupied by the eastward flowing streams 

 was scarcely at all affected and the gorge cutting was confined chiefly 

 to their lower portions, which are now occupied b}'^ the waters of the lake. 

 It is true the main trunk stream entering the head of the bay cut its 

 channel backward well toward its headwaters, but the tributaries from 

 the southwest cut onl}^ shallow trenches in the outer portion of the Rivas 

 plain and none at all in its inner portion. 



The relations of coast lines and divides which prevailed at this period 

 are represented on the outline map (plate 30). The divide between the 

 streams flowing to the Pacific and those flowing to the bay, which, after 

 the ba}^ had been converted into a lake, became the Continental divide, 

 is shown by the broken line nearest the Pacific coast. The length of the 

 streams flowing in opposite directions from this divide is seen to be very 

 unequal. The inequality in length is so great that before the acceleration 

 in corrasion could be felt half way up the courses of the longer east- 

 flowing streams it had caused a deepening of the entire channels of the 

 shorter Pacific streams. With such advantages, the shorter streams began 

 an active conquest of drainage area from those less favorably located on 

 the east of the divide. The result was that at one point, where the ad- 

 vantages of the Pacific stream were most decided, the divide between con- 

 tending streams was pushed east, and successive portions of the eastern 

 drainage were diverted to the Pacific. 



The rapidity with which diff'erent portions of the divide were shifted 

 east depended largely on its relative bight and the length of the con- 

 tending streams. The conditions were evidently most favorable nearly 

 opposite the end of the bay, probably because the soft sedimentary rocks 

 here extended entirely across from the ocean to the bay, while to the 

 north and south there were considerable arefife of harder igneous rocks ; 

 hence the surface had here been well reduced in the preceding period, 

 and was favorably circumstanced for further rapid reduction. 



The stream which suff'ered diversion earliest appears to have occupied 

 the present position of a portion of the Tola, the upper Rio Grande, the 

 Guiscoyol, and the lower Lajas. This stream was probably 5 or 6 times 

 the length of its opponent on the Pacific side, so that the same fall from 

 the divide was distributed over a correspondingly greater distance, and 

 hence had relatively much less than a fifth of the efficiency of the shorter 

 stream. This east-flowing stream had in the preceding period developed 

 a rather broad valley, the upper portion of which lay between the main 



