] 98 APPENDIX. 



The new surface of the area that had been denuded will no longer have the same tempera- 

 ture as when it was covered by a considerable thickness of rock, and the underground 

 isothermal surfaces, down to the focus of beat will be similarly affected ; the wbole mass will 

 have undergone a reduction of temperature proportional to the depth to which the original 

 surface had been cut down, and will bave undergone a corresponding contraction of volume. 

 A similar process will come into operation, but in the opposite direction, in the case of 

 tbe area upon which additional matter bad been laid down. What was before an exposed 

 surface becomes an underground surface, and, in the newly established distribution of 

 heat, its temperature will be raised in proportion to the depth to which it has been 

 covered. 



If the reasoning indicated in the foregoing paragraph be applied to the familiar facts of 

 geology, we get a natural cause almost unlimited in mechanical power. The thickness of 

 successive deposits is known to be very great, and a deposit in one place involves the 

 removal of the same matter from some other area, generally contiguous. In addition to the 

 influence of these fluctuations of heat in producing depressions and elevations of areas, respec- 

 tively of denudation and of deposition, Herschell in his development of the subject, lays great 

 stress upon the effects of the changes of pressure produced by the same process. Whatever we 

 believe the constitution of the earth's crust to be, we must grant some influence to changes of 

 pressure ; the action would be more immediate than that of the changing temperature, and in the 

 opposite direction, tending to depress areas of deposition, and both directly and by re-action, 

 to elevate areas of denudation. By the legitimate application of these two prominent elements 

 of the theory — depression by weight, and expansion by heat — it is possible to give a satisfac- 

 tory or at least a plausible explanation of most phenomena of disturbance. One of the 

 most important inferences of this theory, bearing upon the formation of mountains, is the 

 indication it gives of a position of weakness and of strain between the area of deposition 

 and that of denudation, culminating in fracture, shock, and possibly intrusion or ejection of 

 igneous matter. 



Judged xipon a priori grounds alone this theory is even more complete than that of M. de 

 Beaumont ; both start from this simple fact of a high internal temperature, and the necessary 

 process of slow refrigeration. M. de Beaumont makes this process, and all the results which 

 it entails self-regulating ; whereas Babbage and Herschell propose to maintain the equili- 

 brium by the aid of an equally constant fact — the shifting of materials at the earth's surface, 

 of Which phenomenon M. de Beaumont takes no account. The whole earth is sensitive, as 

 one organism, to any modification of its conditions of temperature. There must be 

 an initial tendency to the production of the world-wide tensions of tbe crust to which 

 M. de Beaumont attributes the production of mountains ; it is also evident that such 

 tensions may be guided and ultimately satisfied by the process to which the other theory 

 appeals. 



The only instance I am acquainted with in which the theory sketched in the preceding 

 paragraphs is applied to explain contortion and elevation of 

 strata is in Mr. J . Hall's account of the Alleghanies or Appala- 

 chians, published in the introduction to Part IV., Vol. 3 of Natural History of New York 

 (1859). The author commences with a review of the distribution of the great palasozoic 

 series in eastern North America. He shows an aggregate thickness of these formations in 



