KNOWLEDGE OF EARTH STRUCTURE 175 



sufficiently powerful? Finally, such forces do exist in nature, 

 and have often been in operation." 



The advanced nature of these conceptions may foe 

 appreciated by contrasting them with those put forth by 

 H. D. and W. B. Rogers on April 29, 1842, before the third 

 annual meeting of the same body (43, 177, 1842) and 

 repeated by them before the British Association at Man- 

 chester two months later. In their own words, the 

 Rogers brothers from their studies on the folds shown in 

 Pennsylvania and Virginia, conceived mountain folds in 

 general to be produced by much elastic vapor escaping 

 through many parallel fissures formed in succession, pro- 

 ducing violent propulsive wave oscillations on the sur- 

 face of the fluid earth beneath a thin crust. Thus actual 

 billows are assumed to have rolled along through the 

 crust. They did not think tangential pressure alone 

 could produce folds. Such pressures were regarded as 

 secondary, produced by the propagation of the waves and 

 the only expression of tangential forces which they 

 admitted was to fix the folds and hold them in position 

 after the violent oscillation had subsided (44, 360, 1843). 

 The leading British geologists De la Beche and Sedg- 

 wick criticized adversely this remarkable theory, stating 

 that they could see no such analogy in mountain folds to 

 violent earthquake waves and that in their opinion the 

 slow application of tangential force was sufficient to 

 account for the phenomena (44, 362-365, 1843). 



H. D. Rogers in the prosecution of the geological sur- 

 vey of Pennsylvania displayed notable organizing ability 

 and persistence in accomplishment, even to advancing per- 

 sonally considerable sums of money, trusting to the state 

 legislature to later reimburse him. Finally, after many 

 delays by the state, the publication was placed directly 

 in his charge and he produced in 1858 a magnificent 

 quarto work of over 1,600 pages, handsomely illustrated, 

 and accompanied by an atlas. It is excellent from the 

 descriptive standpoint, standing in the first class. Meas- 

 ured as a contribution to the theory of dynamical geol- 

 ogy, the explanatory portions were, however, thirty years 

 behind the times. The same hypotheses are put forth 

 in 1858 as in 1842. There is no acceptance of the views 



