230 GEOLOGY. 



once covered this area, but were subsequently removed by erosion 

 before the Potsdam epoch, an alternative for which there appears to 

 be no warrant. Sections 27 and 28 show Middle Cambrian strata rest- 

 ing on Proterozoic or Archeozoic rock, thus indicating that the Middle 

 Cambrian sea covered the localities of these sections, although the 

 Early Cambrian sea did not. Sections 18 to 26 show the presence of 

 VJpper Cambrian beds only, and therefore give some clue as to the 

 extent of the advance of the sea upon ilHe land in the Upper Cam- 

 brian epoch. 



Interpretations. 



Interpreting these sections, it is inferred that in the early part of 

 the Cambrian period the sea covered some parts of the eastern 

 border of the present continent as far sojuth as Massachusetts, that 

 it occupied a narrow, strait-like belt along the Appalachians, ttoat 

 it was also present in the west, but that the great interior was, for 

 the most part, land (Fig. 90). From the same data it is inferred that 

 ^during the Middle Cambrian epoch the sea encroached on the interior 

 from the south and west, reaching western Montana, Arizona, and 

 Texas during the Middle Cambrian epoch (Fig. 96), Still later, in 

 the Potsdam (Saratogan) epoch, the sea gained dominion over the 

 land until, at its close, a large part of the area of the United States 

 was beneath the sea (Fig. 96). 



Theoretically, this result may have been brought about by a rise 

 of the sea, or by a lowering of the land, or by both together ; or, chang- 

 ing the point of view somewhat, it may have been brought about by 

 a deep-seated deformation of the earth's body, or by superficial changes, 

 whether gradational or deformational, or by both combined. It may 

 not be best to try to reach final conclusions where the subject is so 

 intricate and the data so imperfect as in this case, but it is well to 

 weigh the considerations that favor one or another of the possible 

 participating agencies and to try to discriminate between the work 

 of the inevitable agencies and that of the more or less uncertain ones. 



There; is need to discriminate at the outset between two classes of 

 deformation. The one is that which has just been called body defor- 

 mation, whose origin is presumed to be internal, while the other, which 

 will be defined below, is superficial deformation, and more or less inde- 



