Domes and Dome Structure. 215 



The Question of CoMse. — 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 ef- 

 fects of expansion and contraction. When a forest fire 

 sweeps over a rocky hillside the surfaces of rocks are rap- 

 idly heated and thereby expanded. The result is a sort 

 of exfoliation. Flakes of rock, broad in comparison with 

 their thickness, break loose and fall away (plate XXXII, 

 figure 2). Thus the effect of surface expansion is to de- 

 velop partings approximately parallel to the original ex- 

 terior. The effect of contraction is illustrated by the cool- 

 ing 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 con- 

 traction, 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 influence penetrates but a small dis- 

 tance. Secular changes penetrate farther, and may be 

 quantitatively adequate. Secular warming after glacia- 

 tion may have been a vera coMsa, but its discussion is com- 

 plicated by the fact that the dome structure, or at least its 

 principal part, antedated a large amount of glacial ero- 

 sion. If the structure originated with Pleistocene climatic 

 changes, the changes must have pertained to an early 

 epoch of glaciation. 



