Tertiary equator, from the Atlas over the Alps and the Caucasus to the Himalayas. Folding on the anterior margin of drifting blocks seems at first sight to offer difficulties in comprehension, since the sima should surely be the more liquid and the sial the more rigid material. But we must remember our comparison between sealing-wax and wax. Thus if the sial block can be considered as solid, like wax, it will nevertheless be folded when the forces of displacement exceed a certain value. The sima certainly gives way and flows as sealing-wax does—but an exceedingly long period of time is needed for that.
The kinds of folding mentioned—at the front margin and at the equator—correspond on the whole to the two movements of the continental blocks, their westerly drift and their drift from the poles. That this rule holds for more ancient periods, in particular the Carboniferous, is shown by the old folds which form the basis of the Andes, as well as the Carboniferous mountain-system which can be followed along the equatorial zone of that time from North America across Europe towards Eastern Asia.
It can frequently be seen that the parallel-folded chains of a mountain system lie in echelon. If such a folded chain be followed, it is found that sooner or later it comes out on the margin of the mountain system and finally disappears, whereupon the next inner chain forms the margin, disappearing likewise some distance farther, and so on. This is the case when the two blocks do not move directly towards each other, but have a shearing movement though with a direct component. The effect of the different movements of the blocks relative to each other may be in a general way illustrated by Fig. 33, in which the left block is supposed to be fixed and the right to move. If the movement be directed at right angles
