On examining the rocks of a mountain range the geologist is impressed by the fact that work has been done in folding, faulting, and uplifting them. Work implies force, and the investigator is led to inquire what forces are available and competent to produce the stupendous effects which the study of mountains and continents discloses.
In modern geology current thought appeals almost exclusively to gravity to answer this question. The various aspects of the Contraction theory of mountain-building invoke horizontal thrust, but attribute that thrust simply to the gravitative settling of the crust on a cooling nucleus or to the weight of heavier wedges pressing against lighter segments. Isostatic theories rest on the concept of unequal loading, it being postulated that there is a deep-seated underflow from a more heavily loaded area toward an unloaded area, and that the undertow drags the superficial crust with it. In all . . .