76 
OROGENY AND EARTH’S ROTATION 
large value of the vertical component. This is so great that its 
replica in diagram suggests the large vertical exaggeration per¬ 
mitted in blackboard demonstration rather than the normal scale 
of nature. In the experiments this is readily or partially explained, 
however, by the fact of the relatively enormous toughness of the 
materials used. This is so superior to that found in nature that 
relief of tangential stress through faulting is not possible, and 
the effect becomes cumulative to such an extent as to appear 
diagrammatic. 
The notable central infolding presented in the laboratory in spite 
of rigid retention by peripheral banding discloses in nature a new 
importance of gravity, and accounts for a host of phenomena 
which have always remained inexplicable. 
The recent experiments with roll-paper appear to indicate that 
the overturn fold is a product of special conditions. Only when 
the roll is especially loosely wound are multiple flexures developed 
to notable extent. In nature analogous conditions obtain where 
there is alternation of hard and soft strata the latter being repre¬ 
sented by shales in which intermolar movement corresponds essen¬ 
tially to normal rock-flow. 
The development of the simple, open fold into a closely ap- 
pressed flexure the peak of which is finally overturned appears 
to take place only in the center of the anticlinorium where mass 
motion is greatest. It is usually accompanied by medial infold¬ 
ing, or down-folding, and often faces a similar overturn from the 
opposite limb (figure 6). The surprising feature in experiment 
Figure 6. Kvolution of the Overturn Plication 
is again the relatively enormous vertical displacement which is 
involved. In one instance the vertical mobility was one-thirtieth 
of the radius. On the scale of the earth this is a fold having an 
amplitude of about 135 miles, or fifty times greater than any 
natural structure of similar character known. Long before the at- 
