234 The American Geologist. October, 1892 
a very simple experiment. Take an ordinary large note-book, say an 
inch in thickness, with flexible covers. Rule carefully a series of jiaral- 
lel lines across the edges of the leaves at the top of the ))ook, abcut 
1^ of an inch ajjart, 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 cen- 
tral third of the book, where they arrange themselves into a beautiful 
sheath-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 breadth. It is clear that the wave which the book 
now forms must be regarded as made up of three sections, viz.; a sec- 
tion forming the outside of the trough on the one side, and a section 
forming the outside of the arch on the other, and a central or common sec- 
tion, which may be regarded either as uniting or dividing the other two. 
As this experiment gives us a fair representation of what takes place 
in a geological fold, we see at a glance that the geologist is forced to di- 
vide his fold into three parts— -an arch limb, a trough limb, and a mid- 
dle limb — which last we may call the co-puhi or tbe septum, according as 
Ave regard it as connecting or dividing the other two. Our note-book 
experiment shows us also that in the trough limb and the arch limb the 
leaves or layers undergo scarcely any change of position beyond taking 
on the growing curvature of the wave. But the layers in the central 
part, or septum, undergo sliding and shearing. It will be found also, by 
gripping the unbound parts of the book firmly and practicing the fold- 
ing in different ways, that this septum is also a region of warping and 
twisting. This simple experiment should be practiced again and again 
until all these points are apparent and the various stages of he folding 
process become clear; the surface of the book being forced first into a 
gentle arch-like rise with a corresponding trough-like fall; then stage 
by stage the arch should be pushed over onto the trough imtil the sur- 
faces of the two are in contact and the book can be folded no further. 
In the structure of our modern mountain ranges we discover the most 
beautiful illustrations of the bending and folding of tbe rocky forma- 
tions of the earth-crust. The early results of Rogers among the AUe- 
ghanies, of Lory and Favre in the western Alps, have been greatly ex- 
tended of late years by the discoveries of Helm and Baltzer in the cen- 
tral Alps, of Bertrand in Provence, of Margerie in Languedoc, of Button 
and his colleagues in the western ranges of America, and of Peach and 
Home and others in the older rocks of Britain. The light these re- 
searches throw upon the phenomena of mountain structure will be 
found admirably summarized and discussed in the works of Leconte, of 
Dana, of Daubree, of Reade, of Heim, and finally in the magnificent 
work of Suess, the "Antlitz der Erde," of which only the first two vol- 
umes have yet appeared. 
