THE most generally accepted explanation of the formation of mountains has been that of a slowly contracting earth with a solid crust accommodating itself to a shrinking interior. As the assumed shrinking proceeds, so the crust has to fold and fracture in order to occupy a smaller area. Such movements, particularly folding, would be facilitated by the hot glassy substratum on which the crust is believed to rest. Now it might be supposed on general grounds that wherever the crust was at all uniform in texture some regular pattern of folds or fractures would be formed. Since the floors of the oceans, particularly the Pacific, are believed to be fairly uniform, and to consist largely of basaltic material, these should show the pattern well; but the continents in spite of their more varied constitution should also show something of a regular pattern. Therefore if one could ascertain what would be the pattern of a contracting sphere, then this should be tracable in the tectonic structures of the earth’s crust, and if the thermal contraction hypothesis is the real cause of these structures, in this way it should be possible to apply a crucial test to the validity of the hypothesis.