236 The American Geologist. October, i892 
We see also from our note-book experiment that the final result of the 
completion of the fold is clearly to strengthen up and consolidate that 
part of the crust plate to the local weakness of Avhich it actually owed 
its origin and position. The fold has by its life-action theoretically 
trebled the thickness of that pait of the earth-plate in which it3 dead re- 
mains now lie. If the lateral pressure goes on increasing and the layers 
of the earth-crust again begin to fold in the same region, the inert re- 
mains of the first fold can only move as a passive part of a newer fold: 
either as a part of the new arch-limb, the new trough-limb, or the new 
septum. As each j^ounger and younger fold formed in this way neces- 
sarily includes a more resis ant, and therefore a thicker, broader, and 
deeper sheet of the earth-crust, we have here the phylogenetic evolution 
of a whole family of crust folds, each successive member of which is of 
a higher grade than its immediate predecessor. 
But it very rarely happens that the continuous plate in which any 
fold is imbedded is able to resist the crust creep until the death of the 
first fold. Usually, long before the first simple fold is completed, a new 
and parallel one rises in front of it on the side of the trough limb, and 
the two grow, as it were, henceforward side by side. But the younger 
fold, being due to a greater pressure than the older, must of necessity be 
of a higher specific grade, and the two together form a generic fold in 
common. 
Our present mountain systems are all constituted of several families 
of folds, all formed in this way, of different gradations of size, of differ- 
ent dates of origin, and of different stages of life evolution; and in each 
family group the members are related to each other by this natural gen- 
etic affinity. 
Sometimes the new folds are formed in successive order on one side 
of the first fold, and then we have our unilateral (or so-called unsym- 
metrical) mountain groups, like those of the Jura and the Bavarian Alps. 
Sometimes they are formed on both sides of the original fold, and then 
we have our bilateral (or so-called symmetrical) ranges, like the Central 
Alps. In both cases the septa of the aged or dead folds are of necessity 
all directed inwards towards the primary fold. If, therefore, they orig- 
inate only on one side of the fold, our mountain group looks unsym- 
metrical, with a very steep side opposed to a gently sloping side. If 
they grow on both sides of the original fold, we have Ihe well-known 
"fan structure" of mountain ranges. In this case the whole complex 
range is seen at a glance to be a vast compound arch of the upper layers 
of the earth-crust, keyed up by the material of the dead or dying folds, 
which by the necessities of the case constitute mighty wedges whose 
apices are directed inwards towards the centre of the system. But a 
complete arch of this kind is in reality not a single fold, but a double 
one, with a septum on both sides of it; and it requires two troughs, one 
on each side of it, as its natural complement. The so-called unsym- 
metrical ranges, therefore, which are constituted merely of arch limb> 
trough limb, and septum, are locally the more natural and the more 
common. 
