710 TRAKSACTIOIN'S 01? SECTION C. 



might delei'inlue, in the case of the Alps, for iastance, whether the fretjuent earth- 

 quakes of that chain, which pointed to slipping along lines of fracture, were 

 accompanied by movements of uplift or of depression. Another question which 

 opened out a wide field for observation was the determination of the successive 

 periods of disturbance to which any given mountain range owes its origin. It 

 would be interesting also to ascertain the conditions under which volcanic vents 

 are at last opened along a chain of mountains. As investigations advance doubt- 

 less many new features of structure may be expected to arise, which wiU engage 

 the attention of geologists for many generations to come. 



Professor Sollas remarked that all doubts he might have felt as f o the existence 

 of recumbent folds on the scale asserted by French and Swiss geologists had been 

 dispelled by a careful examination of the pre-Alps, made under the guidance of 

 Professor M. Lugeou. The structure was obvious and the interpretation was 

 confirmed by the superposition of the Piedmontese on the Helvetian facies. The 

 question had been much simplified by the new views advanced by the Presi- 

 dent, which completely explained one of the greatest difficulties of the subject. 

 The geometrical properties of the fold were worthy of some consideration ; it was 

 obvious that the volume within the fold increased with its growth up to a stage 

 at which the middle limb was nearly vertical, and up to this point the internal 

 pressure diminished. Beyond it, further advance of the hinder limb produced a 

 nattening-out of the fold and a diminution of internal volume, which led to tlie 

 extraversation of molten material from the region of the hinder limb, i.e., on the 

 concave side of the mountain arc. "When the three limbs of the fold were reduced 

 to parallelism a second fold might arise behind it, to pass through the same stages, 

 and this might be repeated several times. AVhen the middle limb was pushed 

 past the vertical the conditions would render an independent movement of the 

 hinder limb very possible, and this might advance as a sheet over great distances ; 

 superimposed sheets, such as occur in the Alps, might thus be formed. The piled- 

 up folds would, as the President had shown, blanket over the deeper region so 

 elfectually that the geotherms might rise through a great interval. A highly 

 healed region beneath the mountain folds might therefore be expected, and this 

 might account for the lower value of gravity met with in the neighbourhood of 

 mountain chains. The approximation of remote areas brought about by folding 

 involved the transference of viscous material fi'om one region to another ; bulging 

 up, though it occurred, would not account for tlie whole of this. It might travel 

 long distances. The thrusting which built up tbe Alps and Carpathians was felt 

 in the British Isles, where it produced the posthumous folds of the Weald and 

 splintered tbe crust of Scotland and the North of England, as well as parts of 

 Ireland. The cracks then produced afiForded a passage for underljing lava, which 

 itself might also have originated under the Alps. The upward growth of a 

 mountain chain possibly represented only a balance of movemjnt, for a subter- 

 ranean draining-off of molten material would produce a continual lowering of the 

 base. 



Given the deformatioual figure of the earth, the position of the mountain 

 chains, on Professor Joly's theory, follows from it, since they will arise where 

 .sediments are deposited at the limits of land and water. The.e is much indeed 

 to feuggest that the mountain chains of the past have thus originated as a con- 

 sequence of the varying influence of the diflferential harmonics elicited by Professor 

 Love. 



Professor Cole urged, as Sir A. Geikie had done, the importance of keeping in 

 view the formation of mountain-masses by block-uplift as well as by overfolding. The 

 s^rfy geologists, such aa Scrope, recognised recumbent folds when they attributed 

 mountains to the vertical upthrust of igneous cores and the slipping away of sedi- 

 ments on either side. Gravitational sliding being well recognised in existing theories 

 of Alpine structure, it became important to ask if the older views were really 

 wrong, and perhaps, with E. Reyer, we ought to reconsider the whole theory of 

 lateral thrusting as against that of superficial downsliding. The speaker com- 

 pared the curved front of a mountain chain in process of formation with the 

 advancing front of a land-slide, and a.skedfor a tolerant examination of Ampferer's 



