THE CAMBRIAN PERIOD. 235 



each of the great periods of body deformation. It is believed that 

 during great deformations the land segments are squeezed between 

 the ocean segments, and forced up relatively (Vol. I, pp. 542-551), 

 and that the perpetuation of the continents in the face of gradation, 

 spreading, etc., is dependent on this periodic rejuvenation. As the 

 periods between these rejuvenations of the continents appear to have 

 been long, the lateral creeping action must have been slow and limited 

 in amount relatively; yet it may have counted for something in the 

 lowering of the surface, and in the spread of the sea. (20 The faulting 

 of the land areas is predominantly of the tensional type, and this type 

 is hence called normal. Just how this type of faulting came to be a 

 dominant one, is an outstanding problem of geology. It may have 

 taken place in part during the periods of continental compression; 

 but the thrust (reversed) faulting is most reasonably referred to these 

 periods, and it is not easy to see how tension faulting could have 

 predominated during a period of dominant lateral thrust. If tension 

 faults did not occur abundantly during periods of compression, they 

 are chiefly to be referred to the intervening periods of quiescence or 

 relaxation, and the spreading of the continents would seem to fur- 

 nish appropriate conditions. Normal faulting may therefore finally 

 prove to be an evidence of a glacier-like, but extremely slow, creep of 

 the continental masses. (3') The upper portion of all formations com- 

 posed of non-plastic rocks is affected by crevices, and these manifest 

 a nearly universal tendency to gap. Taken altogether, the amount 

 of gaping thus indicated is very considerable, even when allowance 

 is made for enlargement by solution or any demonstrable shrinkage. 

 Cracking and spreading of this kind is what might be expected if the 

 deeper parts have crept laterally under pressure, just as similar cracks 

 open and widen on the surface of asphalt when it slowly spreads under 

 its own weight. Without affirming that this is the explanation of 

 crevicing, or that it is evidence of continental spreading, the mutual 

 fitness of the two is worthy of consideration. 



(b) Mutual adjustment of continental and oceanic segments. — Con- 

 tinental spreading would be operative, as seen above, even if the base 

 plane on which the continent may be conceived to rest were abso- 

 lutely inflexible. This, however, is not the case, and there is need now 

 to consider whether the continents might not sink and cause the ocean 

 bottoms to rise by reciprocal action, like the two arms of a balance. 



