32 G. K. GILBERT — DOMES AND DOME STRUCTURE 



only by dome structure tend to survive general degradation, and often 

 to stand forth as prominent hills. 



The Question of Cause 



In the effort to pass from the general phenomena of dome structure to 

 its cause, I have found instruction in a comparison of the disrupting 

 effects of expansion and contraction. When a forest fire sweeps over a 

 rocky hillside the surfaces of rocks are rapidly heated and thereby ex- 

 panded. The result is a sort of exfoliation. Flakes of rock, broad in 

 comparison with their thickness, break loose and fall away (plate 4, 

 figure 2). Thus the effect of surface expansion is to develop partings 

 approximately parallel to the original exterior. The effect of contraction 

 is illustrated by the cooling of a lava stream or dike. The cooling and 

 contraction begin at the surface, and there develop a plexus of cracks, 

 which are propagated downward or inward as cooling proceeds. These 

 cracks are normal to the surface, and they separate the rock into normal 

 columns. Comparing dome structure with these familiar types, it seems 

 evident that it should be ascribed to expansion rather than to contrac- 

 tion, and we are led to inquire what natural process or processes may 

 have expanded the Sierra granite at the surface. 



Heating is naturally the first to suggest itself. Diurnal and annual 

 changes of temperature may be dismissed at once, because their influ- 

 ence penetrates but a small distance. Secular changes penetrate farther, 

 and may be quantitatively adequate. Secular warming after glaciation 

 may have been a vera causa, but its discussion is complicated by the fact 

 that the dome structure, or at least its principal part, antedated a large 

 amount of glacial erosion. If the structure originated with Pleistocene 

 climatic changes, the changes must have pertained to an early epoch of 

 glaciation. 



A second process developing expansive force is weathering, and here 

 again future investigation may discover a true cause ; but to cursory 

 and inexpert observation the granites of the Sierra in the glaciated dis- 

 trict appear to be unaltered. 



A third process— one as to which we have no direct knowledge — is 

 dilatation from unloading. When the granite came into existence by 

 the cooling of the parent magma it was buried under a deep cover of 

 older rock. Because of that cover it was subject to compressive stress, 

 and that compressive stress was of course balanced by internal expan- 

 sive stress competent to cause actual expansion if the external pressure 

 were removed. As in course of time the load was in fact gradually 

 removed, the compressive stress was diminished and the expansive 



