138 GROVE KARL GILBERT— DAVIS [Memoir ^™xi; 



the concave banks on the right and a little withdrawn from concave banks on the left; and thus 

 a right-handed shift for the whole course should bo gradually effected. He calculated that 

 for the Mississippi "the selective tendency [to deflect the thread of maximum velocity] to- 

 ward the right bank is . . . nearly nine per cent greater than toward the left." To test his 

 views he sent an experienced observer to Long Island, to examine the valleys there which E. 

 Lewis had described in 1877 as exhibiting unsymmetrical cross profiles due to the deflective 

 force of the earth's rotation; and the results thus secured were briefly summarized: 



The south side of Long Island is a plain of remarkable evenness, descending with gentle inclination from the 

 morainic ridge of the interior to the Atlantic Ocean. It is crossed by a great number of small streams which 

 have excavated shallow valleys in the homogenous modified drift of the plain. Each of these little valleys ia 

 limited on the west or right side by a bluff from 10 to 20 feet high, while its gentle slope on the left side merges 

 imperceptibly with the plain. The stream iD each case follows closely the bluff at the right. There seems to 

 be no room for reasonable doubt that these peculiar features are, as believed by Mr. Lewis, the result of ter- 

 restrial rotation. 



Although he quoted Buff as according "a more important influence to the prevailing 

 winds than to the rotation of the earth," he seemed to dissent from that view; and said with more 

 definiteness than usual in announcing a scientific conclusion: "It is my present intention to 

 maintain the sufficiency of the cause" — the deflective force arising from the earth's rotation — 

 for the deflection of rivers. Yet it has been later shown by a study of detailed maps of the 

 Lower Mississippi on which the river course is shown at the time of two surveys separated by 

 an interval of about 13 years, that the displacement of the later course with respect to the 

 earlier is clearly to the east or left, as if because of the winds prevailing, and not to the west or 

 right, as the earth's rotation would have it. It may therefore be inferred that the wind ex- 

 ercises a stronger effect on large rivers than on small streams; but it does not follow that no 

 other control than the earth's rotation has determined the asymmetry of the valleys eroded 

 by small streams of Long Island. The slant of rain in west winds may be important there. 



AGE OF THE EQUUS FAUNA 



Another communication was made to the academy in 1886, on the "Age of theEquus fauna," 

 a subject which is treated in the ninth chapter of the Bonneville monograph, later issued, and 

 which may surprise some readers by the suggestion it gives that Gilbert was making an excursion 

 into paleontology; but as a matter of fact his chief object appears to have been to teach a lesson 

 that he thought paleontologists ought to learn from physiography. 



The need of the lesson arose as follows : Few fossils had been found in the Bonneville beds, 

 but the deposits of a smaller and not distant extinct lake had been discovered to contain a rich 

 assemblage of mammalian fossils, to which the name Equus fauna had been given, and for 

 which a Pliocene date was given by a leading vertebrate paleontologist of the time. But 

 Gilbert was persuaded by the freshness of the Bonneville shore fines that they could not be 

 Pliocene, and by the freshness of the deposits in the other smaller basin that they must be 

 substantially contemporary with the Bonneville deposits; the latter opinion was held with all 

 the more confidence because the deposits in each basin implied a former moist climatic period 

 which must have been contemporaneous in the two districts. He therefore undertook an 

 examination of the principles of correlation by which the Equus fauna had been and should be 

 dated, and showed that for the case in hand the resemblance of fossil forms, in America and 

 Europe, in view of which the American fauna had been made the equivalent of the European 

 Pliocene, was a less trustworthy guide than the physical contrasts between the surface features 

 of the formation that contain the American fauna and those of the European Pliocene, and that 

 in view of these contrasts the two formations should be regarded as of different ages; for while 

 the original limits of the latter are to-day hardly identifiable and their original surface is obsolete 

 or obsolescent, the shore lines of the former are fresh and their original surface is unworn. 

 Hence, in spite of the paleontological argument, Gilbert maintained that the Equus fauna, like 

 the Bonneville deposits, should be classed as Pleistocene and referred to the later half of the 

 later Glacial epoch. 



