TRANSACTIONS OF SECTION 6. 709 



Llgii tetaperatuve would find themselves unacted upon by movement of trans- 

 lation, seeing- that the medium around them would he capable of exerting 

 hydrostatic effects only. These effects would doubtless result in the bulging 

 and elevation of the accumulatiDg folds above, but could not resiilt in translatory 

 horizontal motion in one direction more than another. 



This hydrostatic condition, however, must gradually give place in an upward 

 direction to conditions of rigidity under which the compressive forces would dis- 

 place the folded strata. In this matter the physical properties of calcareous 

 .strata must play a part. Such rocks would not assume a state of plasticity 

 as a thermal effect so readily as siliceous materials, while possessing an inherent 

 plasticity of a crystalline character, allowing of folding and bending imder long 

 continued stress. 



If the process of folding was followed through its stages : — the first forma- 

 tion of simple anticlines and syncliues ; the overthrow of the anticlines of these 

 folds under the directed stresses in the crust; their displacement in the direction 

 of the translatory force while the e.^treme depths of the synclines remained com- 

 paratively unaffected by translatory horizontal movements; and it be borne in mind 

 that the upper and cooler parts of the accumulated mass being the more rigid 

 must partake most fully of the translatory displacement, it would be found that 

 the ' d(5ferlement,' described by M. Lugeon, must arise as the inevitable result. 



In support of this view the speaker had made calculations of the temperature 

 which must be attained under such a depth of material as Professor Schmidt of 

 Basel had assigned to the accumulated folds above the Simplon region. This 

 was .still nearer the surface than the position of the roots of many of the recum- 

 bent folds. If sufficient time were permitted for radioactive heat to accumulate, 

 there must arise a temperature of from 800° to 1000° at a horizon now approxi- 

 mately coinciding with the mean height of the Alps. The assumption of a viscous 

 temperature at the horizon of the synclinal parts of the folds was, therefore, in the 

 l»ighest degree probable, even if the events took place within a period too short to 

 permit of the equilibrium radioactive temperature being attained. 



Sir Archibald Geikie remarked that the term 'mountain' had rather a vague 

 signification, but ihat from the geological point of view it included two main types 

 of structure. In the first place, and most appropriately, it was applied to chains 

 which, like the Alps, have been ridged up by plication of the terrestrial crust ; 

 in the second place, it was often also used to describe the results of the prolonged 

 denudation of large tracts of ground in which thick masses of marine sediments 

 have been elevated into land without .serious displacement of their original approxi- 

 mate horizontality. The ultimate condition of these upraised plateaux might be 

 such a network of lofty and rugged ridges and deep valleys as .almost to rival tbose 

 produced by the plication of the crust. The cause of the uplift of .such plateaux 

 was one of the moot problems of geology. Possibly some of the suggestions put 

 forward in the President's Address might ultimately furnish its solution. 



With regard to mountain chains due to deformation of the crust, opinion 

 had greatly varied as to their structure and the causes that gave rise to tliem. 

 The speaker gave an outline of the liistory of the investigation of the subject, and 

 remarked that it was now generally admitted that such chains were produced, not 

 by vertical uplift, or by what used to be called ' volcanic ' energy, but by tangential 

 movements connected with the earth's secular contraction. The extent to which 

 these movements had plicated rocks had been admirably illustrated from the Alps 

 by Heim. Marcel Rertrand, Rothpletz, Schardt, Schmidt, Lugeon, and others had 

 shown how the folds had been drawn out and piled over each other. These 

 fiattened folds had not infrequently been ruptured, as so strikingly displayed in 

 the north-west of Scotland, and had been pushed over each other, so as to bring 

 up some of the older rocks and leave them lying for many miles above the younger 

 formations. 



The general principles of mountain structure had now been fairly well made 

 out, but there were still many questions to which no definite answer could yet be 

 given. Thus we had no means of deciding whether the plication was rapid or 

 slow, or whether it might not still be going on. Delicate geodetic observations 



