194 



SCIENCE. 



[Vol. I., No. 7. 



grotesque. Let us consider it a moment. 

 Silurian fossils have been found, not only in 

 arctic and temperate regions, but within the 

 tropics. By a slight exaggeration of the pos- 

 sible conditions of animal life we may admit 

 that the general climate of the earth was then 

 50° C. warmer than at present. The lowest 

 estimate that has been offered from the geo- 

 logic or the astronomic stand-point for post- 

 silurian time is five million j-ears, which gives 

 us a fall in temperature of one-thousandth of 

 a degree in each centur3^ Can it be that Pro- 

 fessor Whitney thinks a change in temperature 

 of one-thirtieth of a degree was sufficient to 

 degrade Arabia from a centre of civilization 

 to a desert? and to rob successively Persia, 

 Greece, and Italj', of the prestige of empire? 

 Has a change of one-hundredth of a degree 

 so modified the climate of Greenland as to 

 nearly' depopulate it? Can it be that the same 

 change has perceptibly modified the distribu- 

 tion of cultivated plants in France? Has a 

 change of the two-thousandth part of a degree 

 caused the Alpine glaciers to recede several 

 thousand feet? and the Lake of Valencia to 

 lay bare broad tracts for cultivation? And, 

 finally, was it worth while to make a serious 

 investigation of the thermometric data of the 

 past centurj' in the hope of detecting a change 

 of the thousandth part of a degree? 



TEEEACES AND GEAVELS. 



In one place or another our author states 

 correctly all the fundamental principles of the 

 action of rivers in erosion and deposition ; 

 but a strange fatality attends his application 

 of them. 



It is a conspicuous fact, that running water, 

 under some cli'cumstances, erodes its bed, and 

 that, under other circumstances, it builds up 

 its bed by deposition. The conditions which 

 directly determine the performance of the one 

 or the other of these functions are load and ve- 

 locily. We may define the load of a stream as 

 the ratio of its transported ddbris to the volume 

 of its water. With a given velocitj' a stream 

 is able to transport a certain load : an increase 

 of load leads to deposition ; a decrease, to ero- 

 sion. Conversely, to transport a given load 

 a certain velocity is required : an increase of 

 velocitj- leads to erosion ; a decrease, to dep- 

 osition. Under ordinary circumstances the 

 load of a stream at flood-stage is not subject 

 to great variation ; so that the determination 

 of deposition or erosion is usuallj' due to ve- 

 locity. Velocity is a function of grade and 

 volume. An increase in the angle of slope 

 increases the velocity and tends to make a 



stream erode ; a decrease in the angle of slope 

 tends to produce deposition. An increase in 

 volume gives a greater velocity and tends to 

 induce erosion ; a decrease in volume dimin- 

 ishes velocitj' and tends to induce deposition. 



It follows from this, that a stream which 

 flows with so little velocitj- as to form a de- 

 posit in its vallej' maj^, by an increase of vol- 

 ume, be made to excavate its channel more 

 deeplj', and thus abandon its old flood-plain, 

 leaving a portion of it as a terrace on the side 

 of its vallej'. If, therefore, a stream be found 

 bordered with terraces, and if there be good 

 reason for the belief that the inclination of 

 the vallej' through which it flows has not been 

 changed, it is proper to infer that its volume 

 was formerlj' smaller. Bj' drawing the oppo- 

 site and erroneous inference, Whitnej' has been 

 led to see evidence of swollen streams — and 

 therefore of excessive precipitation — where, 

 in realitj', none exists. In point of fact, 

 river- terraces are nearlj' alwaj's produced bj' 

 orographic changes ; and it maj' be doubted 

 whether there are anj' localities where the ef- 

 fect of orographic movements can be so far 

 eliminated as to permit fluctuations in precipi- 

 tation to be inferred from river-terraces. 



If Whitney had escaped this error, it is pos- 

 sible that he might not have been drawn into a 

 studj' of geologic climate ; for it enters into 

 his original discussion of the auriferous grav- 

 els. He there infers that the pliocene rivers 

 were large, because they deposited their load 

 high up on the flank of the Sierra ; and that 

 the modern rivers are relatively small, because 

 thej' have carved canons in the same region. 

 It may, indeed, be true, that the pliocene pre- 

 cipitation and streams were relatively great ; 

 but these facts, so far as thej' have anj' bearing, 

 point in the opposite direction. 



If, however, we dismiss the idea that the 

 behavior of these rivers was dependent upon 

 thSir volume, we can find a more plausible ex- 

 planation of the phenomena by referring them 

 to change of inclination. If the inclination 

 of the western flank of the Sierra was exceed- 

 inglj' gentle in pliocene time, it would be nat- 

 ural for its streams to form deposits on the 

 lower slopes ; and if afterward an elevation 

 occurred, increasing this inclination, the habit 

 of the streams would be reversed, and the 

 caHons we see would result. That such a 

 change in inclination has actuallj' takisn place 

 is rendered probable bj' other considerations. 

 In the first place, the western face, which is 

 far broader than the eastern, is, as described 

 bj' Whitnej' and others, an inclined plain, in- 

 terrupted onlj- by the narrow canons of the 



