318 SLUDIES FOR STUDENTS 
limbs of the folds (Fig. 62). So quick is the change that the 
folds may be said to have corners, where the beds are bent in a 
circular fashion almost within their own radius. In the more 
closely compressed folds the beds constituting opposite limbs of 
the folds are overturned in opposite directions, thus producing a 
true fan fold (Fig. 60). It is clear that the material of the 
domes partly escaped the thrusts which were transmitted in the 
solid rocks below. This thrust from both directions pressed the 
lower parts of the limbs closer and closer together, while the 
rigidity of the partly free dome above prevented the upper part 
of the legs from following, and thus the limbs were overturned 
in opposite directions. It is probable that the folds in the Jura 
represented by Fig. 6 were not very deeply buried, and that had 
the material been much deeper the more regular form of fan fold 
shown by Fig. 9, and characteristic of the Alps, would have been 
produced. It may be suggested that the Jura and the Alps 
belong to the same great geological province, and since the 
types of folding are the same in both mountain ranges it is not 
improbable that if the Jura were uplifted sufficiently and more 
deeply denuded the ordinary fan-shaped fold of the Alps would 
be revealed. 
It follows from the above that the mechanics of the forma- 
tion of fan-shaped synclines are not- the same in all respects as 
those of the anticlines. It can hardly be assumed that synclines 
are of such magnitude that the lower parts reach a level in which 
the thrusts are less than at a higher level. In other words, it 
cannot be assumed that the lower part of the trough of a syn- 
cline is under less lateral compression than the center of the 
fold. This may, however, be the case if a ‘“‘level of no strain” 
is so near the surface as two miles. Even if this supposed level 
is not at a greater depth than seven or eight miles, Davison’s 
later estimate, the thrust may be considerably less at the deeper 
parts of the fold than at the places of greatest lateral force. We 
therefore do not know whether the first and probably the most 
important cause of the production of fan-shaped anticlines— 
difference in amount of thrust—may also apply to the produc- 
