238 GEOLOGY. 



and may have notably modified it locally; but, on the whole, because 

 they aided in lowering the continents, they assisted in the general 

 transgression of the sea, the great fact to be explained. 



If these agencies are adequate, it is unnecessary to appeal to the 

 more occult and periodic forces that affect the profounder deforma- 

 tions of the earth. More particularly is this true because, as already 

 noted in passing, the usual effects of these profounder agencies are of 

 the opposite character. Their normal work appears to be to deepen 

 the ocean basins, to withdraw the epicontinental seas, to compress 

 the continents laterally, and to increase their protuberances. 



The general tenor of these considerations, taken in connection with 

 the sedimentary record of the period, favors the view that from the 

 beginning to the close of the Cambrian, there was general quiescence, 

 so far as the profounder phases of bodily deformation were concerned, 

 and that gradation (degradation of the land, and aggradation of the 

 sea bottom) was a principal cause of the transgression of the sea, and 

 that the transgression thus brought about was attended and modified 

 by superficial war pings and deformations. These deformations were, 

 for the most part, of the milder kind, and due to a series of minor re- 

 adjustments springing largely from previous diastrophism, together 

 with incidental causes of a superficial nature. Locally, they seem to 

 have been somewhat pronounced, as indicated by the great local thick- 

 nesses of the Cambrian sediments, but as will be seen later (p. 257) the 

 common interpretation of these great thicknesses may be called into 

 question, at least to the extent of notably reducing the amount of 

 deformations which they have been assumed to imply. 



The base-leveling movement, attended by further sea-transgres- 

 sion, continued to be the dominant feature of the earlier portion qf the 

 next period. 



Basis for the Subdivision of the Cambrian. 



We have now to inquire the means by which the Cambrian strata 

 may be recognized, and further the means by which the Lower Cam- 

 brian may be distinguished from the Middle, and the Middle from the 

 Upper. In other words, how is it known that the strata shown in 

 sections 18-26, of Fig. 96, are Cambrian, and how is it known that they 

 are Upper Cambrian, as distinct from Lower and Middle. 



Stratigraphic relations. — From what has gone before, it is evident 



