THE CAMBRIAN PERIOD. 237 



moval of matter from the protruding surfaces, and its deposition about 

 the borders of the land, would have disturbed the balance and caused 

 some tendency to warping; for even if the body of the earth be too 

 rigid to be sensibly influenced by such changes, the local loading, where 

 deep deposits were made, would somewhat compress the rock beneath, 

 and the unloading, where the surface was cut away, would permit their 

 expansion. Such effects would obviously be limited if the crust is 

 inflexible to such moderate stresses; but the crust is sometimes held 

 to be responsive to very moderate loading and unloading. The general 

 result of changes of level brought about by this cause would, on the 

 whole, be opposed to submergence of the land. 



(d) Adjustments due to thermal changes resulting from gradation. — 

 The unequal erosion and deposition of the period tended to imequal 

 loss of heat, and hence to unequal contraction, and so contributed 

 slightly to superficial warping. The general effects of changes brought 

 about in this way would have been opposed to those brought about by 

 isostatic adjustments due to gradation. 



(e) Adjustments due to thermal changes incident to diastrophism. — 

 In the course of the distortion of the crust attending a great period of 

 diastrophism, such as that which preceded the Cambrian period, un- 

 equal temperatures were undoubtedly developed in the compressed, 

 contorted, and disrupted masses, and these tended to equalize them- 

 selves by distribution during the following period of quiescence, and 

 this doubtless resulted in some gentle warping. This may have been 

 a contributing cause to submergence. 



(/) Adjustments due to easing of stresses. — So, too, in the process 

 of distortion, unequal stresses must certainly have been developed, 

 and these would tend to ease themselves by slow changes as time went 

 on, and this also contributed to mild warpings and slight deformations. 

 The general tendency of such changes was probably toward a lowering 

 of the elevated segments of the earth. 



Without attempting to make the list entirely exhaustive, it may be 

 remarked that here is a group of changes which had their origin, for 

 the greater part, in previous diastrophism, and which probably con- 

 tinued to slightly modify the configuration of the surface for a long 

 time after the main diastrophic action had ceased. They may have 

 cooperated with the gradational process through the Cambrian period. 

 They may have interfered with the base-leveling process in some measure, 



