44 Mr. E. W. Hilgard on some points in 



twenty-five miles of sedimentary rocks is not anywhere (on the 

 continental areas at least) actually superimposed vertically upon 

 the crust, and hence that it is not unreasonable to assume 

 that a pressure sufficiently great to produce fusion may never 

 occur within the limits of the sedimentary strata, albeit other 

 manifestations of subterraneous thermal action may not be 

 wanting. It is true that, on the whole, Mallet's memoir leaves 

 upon the reader's mind the impression that he seeks the source 

 of volcanic action at depths sufficiently shallow to justify in a 

 measure the objection raised by Forbes, although he ex- 

 pressly declares that, with our present data, the determination 

 of the points at which the maximum of crushing-effects occurs 

 is impossible. 



Similar considerations apply to the objection raised by F. W. 

 Hutton (Nature, Nov. 27, 1873), that " faults show no heating- 

 effects, even where considerable crushing has taken place." 

 The pressure under which the faulting occurred may have been 

 inadequate, in the cases coming under our observation; but 

 above all, time is a most essential element in this connexion. 

 No matter how great the dislocation or crushing, no great in- 

 crease of temperature can occur if it takes place slowly, however 

 great may be the quantity of work performed, or of heat pro- 

 duced. And very many, if not the majority of extensive faults 

 actually occurring, show evidence of having been formed without 

 cataclysmal disturbance. 



Among the other points raised by Hutton (loc. cit.) there are 

 several which are at once disposed of by a perusal of the original 

 memoir. There are others of some weight. That " lines of 

 least resistance once chosen must remain," is doubtless true in 

 a very wdde sense ; and in that sense this is scarcely at variance 

 with observed facts, since the lines of weakness along the bor- 

 ders of continents are still those which exhibit volcanic activity 

 (and earthquake phenomena) most frequently. But in the fold- 

 ing and upheaving of strata by tangential thrust the question 

 of equilibrium must often*of necessity be very delicately balanced, 

 depending as it does upon the vertical pressure of the masses, 

 their nature, dislocation, subsequent consolidation, igneous 

 effusions from fissures, &c. Lines of weakness as to rigidity 

 may thus easily acquire sufficient static resistance to cause a 

 subsequent yielding to take place at some distance from the 

 original axis, as is exemplified in the formation of successive 

 parallel ranges. What is true with regard to the formation of 

 folds is equally so as concerns the settling down of the crust- 

 fragments in consequence of interior contraction. Each frag- 

 ment as a whole may remain as such, being only, as it were, 

 abraded at its circumference. But it is only necessary to have 



