Davis — Bearing of Physiography upon Suess'' Theories. 271 



of France may be mentioned as smaller and larger examples in 

 Europe ; the plateaus of northern Arizona are examples in this 

 country. Graben are the reverse of horsts, being fanlt-bounded 

 areas that stand below their surroundings. The valley of the 

 middle Rhine is a famous European example : the troughs of 

 certain of the great African lakes, described by Gregory as 

 " rift-valleys," are also graben. According to Suess' analysis 

 of the problem, the movements which produce horsts and 

 graben " are easily explained, in the absence of tangential 

 movements, by a yielding of the support and by the force of 

 gravity. Everything of this kind that one observes is only a 

 variation on passive settling or sinking" (Das Antlitzder Erde, 

 i, 165; French translation, i, 162). "Great plains may sink 

 down ; as soon as their support yields, they obey the action of 

 gravity ; but we know no force capable of uplifting, unequally 

 and locally, mountainous masses situated side by side " (ibid., 

 i, 736 ; i, 775). He continues, there are two facts which we 

 cannot escape: the first is that "large areas have simply 

 sunk down under the influence of gravity. The second is that 

 no force is known capable of uplifting numerous great and 

 small mountainous masses vertically and independently, between 

 two plane surfaces, and of sustaining them in this uplifted 

 position permanently, in spite of gravity " (ibid., i, 741 ; i, 782). 

 With all the respect that one must feel for the erudition of 

 such a master of the geology of the whole earth as Suess has 

 shown himself to be, the conclusions indicated in the above 

 extracts do not seem to me to be proved. The faults by which 

 horsts and graben are bounded truly show a differential move- 

 ment, more or less nearly vertical, but the means of determin- 

 ing, independently of all theory, which mass went up or which 

 mass went down are, to say the least, obscure. The observed 

 facts of dislocation taken alone are consistent with various 

 suppositions as to the movement of the adjoining masses : both 

 may have moved upwards, one more than the other ; both 

 may have moved downwards, one more than the other; one 

 may have stood still, and the other may have moved ; both may 

 have moved, one upward, the other downward. It is not satis- 

 factory to appeal to our ignorance of available forces of uplift 

 and support as a mean of choosing among these alternatives : 

 the operations of the earth's interior are so little understood 

 that we are as much in the dark about their action as they are. 

 JSTor does the accumulation of examples that may be explained 

 by subsidence strengthen the case, unless it is shown at the same 

 time that they cannot be explained by upheaval. The actual 

 movement of faulted masses can be rigorously determined only 

 by relating them to some fixed standard of comparison, and that 

 is no easy task. It seems hopeless in the present state of our 

 knowledge to speak of movements with respect to the earth's 

 center ; for the sea-surface and not the earth's center is our 



