, Notices of Memoirs — Prof, LapworWs Address. 421 



horizontal sheets, lying in regular order one over the other, but which had been 

 subsequently bent up into alternating arches and troughs (i.e. the anticlines and 

 synclines of the geologist), while their visible parts, which now constitute the surface 

 of our habitable lands, were simply those parts of the formation which are cut at 

 present by the irregular plane of the present earth's surface. All those parts of the 

 great arches and troughs formerly occurring above that plane have been removed by 

 denudation ; all those parts below that plane lie buried still out of sight within the 

 solid earth-crust, although in every geological sectioa of sufficient extent it was 

 seen that the anticline or arch never occurred without the syncline or trough— in 

 other words, that there was never a rise without a corresponding fall of the stratum. 

 Yet it is only of late years that the stratigraphical geologist has come clearly to 

 recognize the fact that the anticline and syncline must be considered together, and 

 must be united as a single crust-wave, for the arch is never present without its 

 complementary trough, and the two together constitute the tectonic or orographic 

 unit. T/ie Fold, the study of which, so brilliantly inaugurated by Heim in his 

 ' Mechanismus der Gebirgsbildung, ' is destined, I believe, in time, to give us the 

 clue to the laws which rule in the local elevation and depression of the earth-crust, 

 and furnish us with the means of discovery of the occult causes which lie at the 

 source of those superficial irregularities which give to the face of our globe its 

 variety, its beauty, and its habitability. 



We have said already that this wave or fold of the geologist resembles that of 

 the wave of the physicist. 



Now we may regard such a wave as formed of two parts, the arch-like part 

 above and the trough-like part below. The length of the wave is naturally the 

 length of that line joining the outer extremities of the arch and trough, and 

 passing through the centre node or point of origin of the wave itself, which bisects 

 the line of contrary curvatures. The amplitude of the wave is the height of the 

 arch added to the depth of the trough. Now the arch part of such a wave, if 

 perfectly symmetrical, may clearly be regarded as belonging either to a wave 

 travelling to the right, in which case the complementary trough is the one in that 

 direction, or it may be regarded as belonging to a wave travelling to the left, in 

 which case its trough must be the one in that direction. But as in the case of the 

 sea wave, the advancing slope of the wave is always the steeper, and the real 

 centre of the wave must lie half-way down this steeper slope ; so there is no 

 difficulty in recognizing the centre of a geological fold and its real direction of 

 movement. 



The fold of the geologist differs from the ordinary wave of the physicist, 

 essentially in the fact that even in its most elementary conception, as that of a 

 plate bent by a pressure applied from opposite sides, it necessarily includes the 

 element of thickness. And this being the case, the rock sheet which is being 

 folded and curved has different layers of its thickness affected differently; in the arch 

 of the fold the upper layers of the rock sheet are extended, while its lower layers 

 are compressed. On the contrary, in the trough of the fold the upper layers are 

 compressed and the lower layers are extended. But in both arch and trough 

 alike there exists a central layer, which, beyond taking up the common wave-like 

 form, remains practically unaffected. 



But the geological fold has in addition to length and thickness the further 

 element of breadth, and this fact greatly complicates the phenomena. 



But many of the movements which take place in a rock sheet which is being 

 folded, or in other words those produced by the bending of a compound sheet 

 composed of many leaves, can be fairly well studied in a very simple experiment. 

 Take an ordinary large note-book, say an inch in thickness, with flexible covers, 

 rule carefully a series of parallel lines across the edges of the leaves at the top of 

 the book, about i of an inch apart, and exactly at right angles to the plane of the 

 cover. Then, holding the front edges loosely, press the book slowly from back 

 and front into an S-like form until it can be pressed no further. As the wave 

 grows it will be noticed that the cross lines which have been drawn on the upper 

 edge of the book remain fairly parallel throughout the whole of the folding 

 process, except in the central third of the book, where they arrange themselves 

 into a beautiful sheaf-like form, showing how much the leaves of the book have 

 sheared or slidden over each other in this central portion. It will also be seen 

 when the S is complete that the book has been forced into a third of its former 



