British AssockUion^ Edinburgh Meeting — Lcqncorth. 235 
Looking first at the mountain fold in its simplf st form as that of a 
bent rock-plate, composed of many layers which have been forced into 
two similar arc-like forms, the convexities of which are turned, the one 
upwards and the other downwards, we find in the present mountain 
ranges of the globe every kind represented. We commence with one in 
which the arch is represented merely by a gentle swell of the rock-sheet, 
and the trough by an answering shallow depression, the two shading into 
each other in an area of contrary flexure. From this type we pass in- 
sensibly to others in which we see that the sides of the common limb or 
septum are practically perpendicular. From these we pass to folds in 
which the twisted common limb or septum overhangs the vertical, and 
so on to that final extreme, where the arch limb has been pushed com- 
pletely over on to the trough limb, and all three members, as in our note- 
book experiment', are practically welded into one conformable solid 
mass. 
Although the movements of these mountain folds are slow and insen- 
6iblp,and only effected in the course of ages,80 that little or no evidence 
of the actual movement ( f any single one of them has been detected 
since they were first studied, yet it is perfectly plain that when we re- 
gard them collectively, we have here crust folds in every stage of their 
existence. Each example in itself repiesents some one single stage in 
the lifetime of a single fold. They are simply crust folds of different 
ages. Some are, as it were, just born; others are in their earliest youth. 
Some have attained their majority, some are in the prime of life, and 
some are in the decrepit stages of old age. Finally, those in which all 
three members — arch limb, trough limb, and septum — are crushed to- 
gether into a conformable mass, are dead. Their life of individual 
movement is over. If the earth pressure increases, the material which 
they have packed together may of course form a passive part of a later 
fold, but they themselves can move no more. 
In many cases, due partly to the action of longitudinal pressures, 
tho septum becomes reduced to & plane of contrary motion, namely — the 
over-fault, or thrust-plane, and the arch limb and the trough limb slide 
past each other as two solid masses. But here we have no longer a fold, 
but a fault. 
We see that every mountain fold commences first as a gentle alter- 
nate elevation and depression of one or more of the component sheets of 
the geological formations which nuike up the earth-crust. This move- 
ment is due apparently to the tangential thrusts set up by the creeping 
together, as it were, of those neighboring and more resistant parts 
of the earth-crust which lie in front of and behind the moving wave. 
Yielding slowly to these lateral thrusts the crest of the fold rises higher 
and higher, the trough sinks lower and lower, the central common limb 
or septum grows more and more vertical and becomes more and more 
strained, sheared and twisted. As this middle limb yields, the rising 
arch part of the fold is forced gradually over on to the sinking trough, 
until at last all tliree mt-mbers come into conformaljle contact and fur- 
ther folding as such is impossible. Movement ceases, the fold is dead. 
