THE CLIMATIC THEORY OF TERRACES. 31 



cause the terraces to increase or diininish in size, and even to appear and disappear in 

 accordance with purely local conditions of topography. Moreover, regional warping would 

 cause some streams to be accelerated and others to be retarded, according as they flowed 

 with or against the direction of warping. Therefore its effect would be divided into two 

 classes. In the case of streams which were retarded we should have the kind of terraces 

 discussed in the preceding paragraph. In the many cases where the streams were accel- 

 erated, however, we should expect to find young gorges with terraces of rock along their 

 sides. These, as we have already seen, are not found. If regional warping is the cause of 

 the terraces, a given stream ought to behave differently according to the direction in which 

 it flows. The Santa Cruz River heads in the southern part of the State of Ai-izona. It fii'st 

 flows south for about 20 miles into Mexico, then west around the end of the Santa Rita 

 Mountains, next north for 60 miles, and finally northwest. Terraces of the same type 

 continue from the head clear to the beginning of the northwesterly section, where they 

 finally die out. If any general warping of the crust had taken place, parts of the Santa 

 Cruz would have been accelerated and parts retarded. Therefore we should have portions 

 of the river valley assuming the form of gorges with rock terraces, and other portions where 

 deposition had taken place and gravel terraces had been formed ; and the location of these 

 two types would have nothing to do with the topography of the country in the immediate 

 vicinity. No such condition exists, however, and we seem to be forced to abandon the 

 theory of general warping or tilting, whereby the streams were checked and forced to form 

 deposits and terraces. 



In view of the preceding paragraphs we are led to conclude that, if the terraces are due 

 to a checking of the streams by tilting, the tilting must have been extremely local in char- 

 acter, so that each stream or portion of a stream was affected indi^^dually. Here again we 

 meet difficulties. If some parts of a region were tilted so as to retard certain streams, it 

 is inconceivable that other parts should not have been tilted so as to accelerate the streams, 

 but of this, as has been so often said, we find no evidence. Apparently we must give up 

 the theory of crustal movements, except as a reserve hypothesis to explain exceptional 

 phenomena, or else we must conclude that the interior forces of the earth adapt themselves 

 with the most minute precision to the minor topograpliic features of the surface, and always 

 act in such a way as to produce the same results upon regions of similar topography. 



NATURE AND DISTRIBUTION OF TERRACES ACCORDING TO THE CLIMATIC HYPOTHESIS. 



Turning now to the cUmatic theory of the origin of terraces, we find that it seems to fit 

 all the conditions. Let us take up the matter first from a purely theoretical standpoint. 

 For the sake of convenience, let us assume the same initial conditions as in the preceding 

 discussion, namely, that the streams are engaged in broadening and slightlj^ deepening their 

 valleys. The main streams are supposed to have reached a stage of development where 

 they have formed flood-plains, while the minor streams are without flood-plains and the 

 topography is rugged. Let us assume also that the climate is relatively moist. Under 

 such circumstances the slopes of the mountains will be well covered with forests and with 

 other vegetation ; the streams will be numerous, and will be of fahly constant volume with- 

 out being subject either to excessive floods or absolute drying up, and most of them will 

 discharge into the main rivers, so that much of the sediment which they carry will reach 

 the sea with comparative rapidity. In the absence of any positive knowledge as to the 

 climate of Ai-izona during the glacial period, we can not say positively that exactly these 

 conditions ever prevailed there, but there can be little doubt that such was the case, for 

 similar conditions still prevail among the high mountains. 



If moist conditions, such as have just been described, give place to aridity, many 

 other changes wall take place. The forests and a large part of the other vegetation will die ; 

 the streams will diminish in volume, many will dry up entirely part of the time, and will 



