400 



University of California. 



[Vol. i. 



globe. (3) The indirect data afforded by the Gulf indicate that 

 isostatic (or consequent) movement alone is incompetent to explain 

 the general continental oscillations recorded in the Neozoic depos- 

 its. Thus the Gulf of Mexico yields both maximum and minimum 

 measures of isostasy." 



McGee, like Dutton. distinguishes two kinds of vertical earth 

 movement, viz.: "antecedent and consequent — the first including 

 those great initial movements of debatable cause by which conti- 

 nents are lifted and sometimes deformed or drowned, and the sec- 

 ond including the more restricted movements due to loading and 

 unloading." It is not perfectly clear from his paper, wherein the 

 vital distinction between the two lies, but it is to be observed, that 

 in so far as he tends to limit the isostatic hypothesis to the more 

 local movements, he cuts himself adrift from that general form of 

 the theory which almost all physicists and geologists accept, and 

 shows in this respect an opposite tendency to that exhibited in the 

 paper of Gilbert last quoted from. 



Willis, also in 1892,* speaking of the sediments which were laid 

 down on the borders of the Palaeozoic continent over the present 

 area of the Appalachians, says that the "condition of isostasy pre- 

 vailing in the earth's mass demanded that compensation should be 

 made to the continental area for the load taken from it, and a 

 deep-seated flow was set up landward, a movement sufficient to 

 restore elevation to the continent, which might otherwise have 

 remained at rest." Like Hall, he regards the sediments as the 

 active cause of depression of the earth's crust, although differing 

 from the latter in his explanation of the manner in which these 

 strata were subsequently folded and elevated. 



In 1893^ in a paper on the origin of mountain ranges, Professor 

 Le Conte accepts in the main the general isostatic hypothesis as 

 : formulated by Dutton, and regards the earth's crust as yielding 

 under the weight of sediments previous to the elevation of the latter 

 into mountain ranges, and rising because of lightening by erosion. 

 His discussion of this question is marked by a very obvious con- 



*Michanics of Appalachian Structure, 13th An. Rept. U. S. Geol. Survey, 



pp. 237-280. 



t Origin of Mountain Ranges, Jour. Geol., Vol. I, pp. 543-573. 



