TRANSACTIONS OF SECTION C. 701 



dation; all those parts below that plane lie buried still, out of sight within the solid 

 earth-crust. 



Although in every geological section 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 corresjionding fall of the stratum — yet it is only 

 of late years that the stratigrapbical geologist has come clearly to recognise the 

 fact that the auticline 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, structural, or orographic unit, 

 namely. The 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 to furnish us with the means of discovery of the occult causes that lie at 

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

 variety, its beauty, and its habitability. 



Like the form" of the surface wave of the geographer, the form of 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 the 

 axial 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 curvature. The amplitude of the wave is the height of the arch added to 

 the depth of the trough. 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 maybe 

 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 shore 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 also, in the case of the geological^ fold, 

 there is no difficulty in recognising the centre or in tixing the 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 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 com- 

 pressed, and the lower layers are extended. But in 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. 



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 com- 

 posed 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. Kule 

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

 book, about ^ of an incli apart, and exactly at right angles to the plane of the 

 cover. Then, holding the front edges loosely, press the book slowly fi-om 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-iike form, showing how much the leaves of tlie 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 breadth. It is 

 clear that the wave which the book now forms must be regarded as made up of 

 three sections : viz., a section forming the outside of the trough on the one side, a 



