A CENTURY OF GEOLOGY. '275 



especially by Van Hise, there are many other and even greater c-auses 

 of contraction. It would be out of place to follow the discussion here. 

 The subject is very complex, and not yet completely settled. 



We have g-iven the barest outline of the history of mountain ranges 

 and of the theory of their formation as worked out in the last third of 

 the present century, and, I might add, chiefly by American geologists. 

 So true is this that by some it has been called the '"American theorv." 



(hciUatory movements of the eiirthh crust ove7' %mde areas. — We have 

 already spoken of these as modifying the effect of the ocean-basin- 

 making movements, and therefore now touch them very lightly. 

 These diflfer from the movements producing oceanic basins on the one 

 hand and mountain ranges on the other bv the fact that they are not 

 continuously progressive in one direction, but oscillatory — now up, 

 now down, in the same place. Again, they do not involve contraction 

 of the whole earth, but probably are always more or less local and 

 compensatory — i. e.. rising in one place is compensated by down sink- 

 ing in some other place. Nevertheless, thev often affect very wide 

 areas — sometimes, indeed, of more than continental extent — as, for 

 example, in the crust movements of the Quaternary period or ice age. 



These are by far the most frequent and most conspicuous of all crust 

 movements — not only now, but also in all geological times. If ocean- 

 basin-forming movements are the underlying cause and condition of 

 the evolution of the earth, these wide oscillations, by increasing and 

 decreasing the size and height of continents and changing greatly their 

 contours, have determined all the details of the drama enacted on the 

 surface, and were the determining cause of the varying rates and 

 directions of the evolution of the organic kingdom. These were the 

 cause of the unconformities and the corresponding apparent wholesale 

 changes in species so common in the rockv strata, and which gave rise 

 to the doctrine of catastrophism of the early geologists. These also 

 have so greatly modiffed the contours of the continents and their size 

 by temporary increase or decrease that they have obscured thi^ general 

 law of the steady development of these, and therefore their substantial 

 permanency. 



Although the most important of all crust movements in determin- 

 ing the whole history of the earth, and especially of the organic 

 kingdom, we shall dwell no further on them, because no progress has 

 yet been made in their explanation. This, too, nmst be left to the 

 workers of the twentieth century. 



Tlie pr'mclple <>f isostasij.—T\\^ principle of static e(iuilit)riuin as 

 applied to earth forms was first brought forward (as so many other 

 valuable suggestions and anticipations in many departments of sci- 

 ence) by the wonderfully fertile mind of Sir John Herschel, and used 

 by him in the explanation of the sinking of river deltas luider the 



