An extreme case of folding occurs in the larger folded mountains, where the loop of the fold not only is pushed over but is actually laid flat and thrust out from the centre of the range; recumbent folds of this nature are very frequent on the north of the Alps.
It is evident from the above that folds of any nature whatever occurring in the earth's crust imply that the beds have been compressed. The thrust is nearly always horizontal, but how it originates is not yet perfectly understood. Formerly the thrust was explained by the
Fig. 28. Recumbent Folds (Joly)
shrinking of the earth's nucleus as it slowly gave up its heat, and the solid crust enclosing it was, therefore, forced to wrinkle like the skin of an apple which is drying. There are, however, vast areas, like the Rocky Mountains, which have been in tension, and generally we may say that where there is compression in one part of the globe there is an area of tension compensating for it somewhere else, and the earth as a whole is not shrinking. Investigation into the problem of horizontal thrusts is proceeding on the lines of the investigation of the expansion caused by the sinking of beds owing to the loading of the sea floor by sedimentation. This is always going on, and the expanded beds, helped
