94 National Geographic Magazine. 
surface of the country. While these streams are deepening their 
channels in the Cretaceous cover, which is unshaded with mar- 
ginal contour lines in the figures, their subsequent, autogenetic 
branches are irregularly disposed, because there is no lateral 
variation of structure to guide them; but after a time, the base- 
levelled surface of the buried Triassic beds is reached, as is 
shown by linear shading in the valley bottoms of figs. 4, 5, 6, 7. 
The growth of the subsequent branches then developed, will be 
along the strike of the Triassic softer beds, that is, about square 
to the course of the three transverse streams under consideration. 
The most rapid growth will be found on the branches of the 
largest stream, A, because it will most quickly cut down its 
channel close to the baselevel of the time and thus provide steep 
sloping valley-sides, from which the subsequent branches cut 
backwards most energetically. In due time the main streams 
discover the particularly resistant transverse lava sheets in the 
underlying formation; and then the subsequent branches of the 
largest transverse stream on the up-stream side of the obstruc- 
tions, for example, F and G, fig. 4, will have a great advantage 
over those of the smaller streams. The most rapidly growing 
subsequent branch, G, fig. 5, of the largest transverse master 
stream, A, may grow headwards so fast as to push away the 
divide, X, which separates it from the head of the opposing sub- 
sequent branch, J, of the next adjacent smaller transverse 
stream, CO, and thus finally to capture and divert the headwaters, 
H, of the smaller transverse stream to the larger one, as in fig. 6. 
