THE CAMBRIAN PERIOD. 227 



some repetition of what has been said, will help to make clear the method 

 by which the geography (and therefore the geology) of the successive 

 stages of the Cambrian is made out. If, for example, sections 1 and 2 

 be compared, it . is seen that in both cases the three divisions of the 

 Cambrian system are present, and that in both cases the Lowei 

 Cambrian rests unconformably on the Proterozoic, while the Middle 

 rests conformably on the Lower, and the Upper on the Middle. From 

 these two sections it is inferred that the Cambrian seas covered the 

 two localities where these sections occur, during at least the later 

 portion of the Early Cambrian, during the Middle Cambrian, and dur- 

 ing at least the earlier portion of the Later Cambrian epoch. These 

 sections do not indicate how much of the Upper Cambrian in these 

 two areas has been removed by erosion. From the strict conformity 

 of the three groups of strata, it is probable that the Middle Cambrian 

 beds are present in their full thickness in both sections; that is, that 

 both localities were under water continuously in the Middle Cambrian 

 epoch, and that none of the Middle Cambrian series has been lost by 

 erosion. 



It will be noted that the Lower Cambrian in section 1 is much 

 thinner than that in section 2. This might be explained in either of 

 two ways. (1) In the area of section 1, the Proterozoic (Algonkian) 

 surface may have been above water during a part of the Lower Cam- 

 brian epoch. If this be true, the land at this point must have sunk 

 so as to let the sea cover it before the close of the epoch. If the area 

 of section 2 was under water longer during the early Cambrian epoch, 

 the greater thickness of the Lower Cambrian in this section would be 

 explained. (2) The other possible explanation of the inequality is 

 found in the unequal rates of accumulation of sediments. So far as 

 the sections show, both areas may have been under water during the 

 whole of the Georgian epoch; but if the rate of sedimentation was 

 slower at 1 than at 2, the thickness of strata would be correspondingly 

 less. From the sections it is not possible to decide between these 

 alternative explanations. 



If section 2 be compared with section 3, a somewhat different 

 relation is shown. In the latter, the Lower and Middle Cambrian are 

 both present, while the Upper Cambrian is wanting. This again might 

 be explained in either of two ways. Either eastern Massachusetts 

 rose above sea level after the Middle Cambrian epoch, so that the Upper 



