Page:The Journal of geology (IA journalofgeology21894univers).pdf/99

souri,^[1] and the north branch of the Susquehanna in Pennsylvania; the Seine in northwestern France, and the Moselle in western Germany, may be cited in illustration of this kind of occurrence.

Another case in which a second adolescence is unlike the first is found in regions of tilted structure, where the strata are of diverse resistance, thus giving good opportunity for the development of subsequent streams. In the beginning of the first cycle there are no subsequent streams. All the drainage is constructional (antecedent streams not being now considered). In adolescence, the drainage is chiefly consequent, although subsequent side streams are then beginning to bud forth from the consequent streams. In past-mature stages, the subsequent streams may have acquired a considerable part of the drainage area. Now, if a region of this kind, with consequent and subsequent drainage, is bodily elevated, all the streams are revived; they all cut down new trenches toward the new baselevel. But in this case the revived subsequent streams begin the new work at the same time as the revived consequent streams, and they will go on rapidly in acquiring still more drainage area. Therefore, in the adolescence or maturity of the second cycle, the drainage area acquired by the subsequent streams will be proportionately large; much larger than at the same stage of the first cycle. Much faith may be placed in this deduction. If the drainage of an adolescent region is largely subsequent, and but little consequent, the region may be regarded as almost certainly in a second cycle of development, after a first cycle of well-advanced age.

Illustrative material.—One of the greatest difficulties in the way of teaching physical geography arises from the failure of the student to know what the teacher is talking about. The teacher may have traveled and observed extensively; a large variety of geographical forms are in his memory, ready to be summoned by name when picturing the stages of the deductive