Page:Popular Science Monthly Volume 50.djvu/257

Rh that inclosing this inner sphere is a comparatively thin shell of solid rocks—the passage from the hot and potentially plastic interior to the cold and rigid outer shell being gradual, one merging with the other by insensible gradations.

The crust of the earth rests on the sphere of plastic material within and exerts a pressure upon it. Contraction of the progressively cooling crust also causes pressure to be exerted on the inner sphere. If the pressure of the crust on the material it incloses were equal at all points, the inner mass, except for the effect of rotation, would be a perfect sphere. Variations in the pressure of the crust at different localities might result from several causes, such as unequal cooling in the crust itself, the transfer of material from the inner sphere into or to the surface of the crust, the shifting of material from one locality to another on the earth's surface, etc. Of these disturbing conditions I am inclined, provisionally at least, to ascribe the greatest potency to the effects of erosion, transportation, and sedimentation on the earth's surface, thus lightening certain areas and loading others.

If we conceive of the earth as a sphere without rotation, it is evident, from our present point of view, that it would remain a sphere only so long as the pressure of the crust on the material within was equal at all points. Local variations in the pressure of the crust would deform the inner sphere and result in a change in he shape of the earth. It is not to the general problems of isostasy, as formulated by Dutton, that I wish to direct attention, however, but rather to the results that might be expected to follow should the crust of the earth be broken.

A fracture in the earth's crust would establish a line of weakness, which, so far as the reaction of the crust on the interior is concerned, would be equivalent to a local relief of pressure. Should a fissure reach the highly heated interior, the rocks in its vicinity would become plastic and be pressed into the opening, and tend to widen it both by pressure and by the fusing of its walls. As the magma from a deep source rose in the fissure, the resistance to be overcome would be less and less, thus insuring greater plasticity, and, if it gained the surface, establishing conditions commonly recognized in volcanic eruptions.

Should the plastic rock fail to reach the surface, but cool in the fracture, a dike would be the result. If the fracture terminated above in a region of horizontal stratified rocks, lateral expansion of the magma rising in it might occur, and intruded sheets, laccolite, etc., be formed.

As to the origin of fractures we have but few facts to guide us. It is well known from the study of faults, dikes and volcanoes, that breaks, at least in the superficial portion of the earth's crust, have been of common occurrence. Whether any of these breaks