IGNEOUS INTRUSIONS AND VOLCANOES. 243 



that inclosing this inner sphere is a comparatively thin shell of 

 solid rocks the passage from the hot and potentially plastic in- 

 terior 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 pro- 

 gressively cooling crust also causes pressure to be exerted on the 

 inner sphere. If the pressure of the crust on the material it in- 

 closes 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 shift- 

 ing 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 fol- 

 low should the crust of the earth be broken. 



A fracture in the earth'^i crust would establish a line of weak- 

 ness, 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 condi- 

 tions 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 termi- 

 nated above in a region of horizontal stratified rocks, lateral ex- 

 pansion 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 



