1918.] The N.Z. Journal of Science and Technology. 281 
and swept away. Indeed, it appears certain that a range with a highly 
accidented initial surface would be maturely dissected while still in its period 
of vigorous growth, and long before it had attained its maximum height. 
Not many years ago all the great mountain-ranges of the world which 
are built of folded rocks were believed to have originated as “ fold 
mountains,” and to be now in process of reduction by erosion for the first 
time ; for it was recognized that erosion' must reduce their height, round 
off their peaks, and ultimately destroy their relief. In the language of 
geomorphology, the mountains were believed to be undergoing erosion in 
a cycle introduced by the uplift accompanying or resulting from folding. 
To put it in another way, they were one-cycle mountains. 
The absolute length of a complete cycle (the time required for the 
destruction o'; a mountain-range) is unknown, and even its relative length 
as compared with intervals of geological time is but vagely understood. 
Moreover, it varies enormously with different kinds of mountain-forming 
rocks, and in different climates. While, however, the immensity of 
geological time is being more clearly realized, the efficiency of subaerial 
erosion and the comparative brevity of the cycle of mountain-destruction 
are becoming more apparent; and the intervals since the geological dates 
of the folding in many ranges, though formerly regarded as brief, seem 
now more than sufficient to allow of the dissection and reduction of the 
mountains to their present state. 
In many parts of the world evidence has come to light also which 
proves that mountain-ranges are really dissected plateaux, though com¬ 
posed of folded rocks ; that is to say, they are two-cycle, or perhaps 
multi-cycle , mountains, the region having been worn down by erosion to 
small relief at least once, and possibly more than once, prior to a nearly 
even uplift, which was followed by deep dissection of the plateau so 
formed. Mountains that have originated in this way are recognized owing 
to the preservation of some remnants of the plateau from which they 
have been carved. The initial form in this case, having a level surface, is 
not so quickly eroded away as the tumbled crest of a pile of folds, but 
is encroached upon little by little as dissection of the highland proceeds, 
a few remnants far from the principal rivers perhaps surviving long after 
other parts of the region have been completely dissected to a sea of sharp 
ridges and peaks. It is justifiable to suspect that other mountain-ranges 
in which plateau-remnants no longer survive have also had a multi-cycle 
origin, and this suspicion often receives a considerable amount of confirma¬ 
tion from an accordance of summit-levels, which suggests the restoration 
of a vanished plateau a little above the present summits of numerous peaks 
of nearly even height. 
A list of the important mountain-ranges that have gone through more 
than one cycle of erosion would be so long that it would make tedious 
reading, but it may be mentioned that even the Jura Mountains, so often 
referred to as an example of a system of rock-folds not yet destroyed by 
erosion, are now known to have been reduced to small relief, uplifted, 
and redissected.* 
It is quite conceivable that the mountains formed by the original 
folding on the sites of some of the great ranges of the present day were 
of relatively insignificant height. The piling-together of rock-folds does 
not necessarily form a great protuberance on the earth’s surface, for the 
* E. Bruckner, Notice preliminaire sur la rr.orphologie du Jura suisse et fraru^ais, 
Arch. sci. phys. nat., 4 e ser., tome 14, pp. 633-41, Geneve, 1902. 
