224 GEOLOGY. 



view, the question is, Shall the Proterozoic era be regarded as ending, so far as 

 this region is concerned, (1) when the uplift preceding the Cambrian began, (2) 

 when it reached its maximum, or (3) when the subsequent subsidence was in- 

 augurated? To this question no categorical answer can be given, and formation 

 1 of Fig. 93 may be looked on as representing the time of transition from the 

 Proterozoic to the Cambrian. 



Extent of unconformity at base of Lower Cambrian. — Except in the 

 northwest, the Lower Cambrian is generally unconformable on deformed 

 beds of Proterozoic age, but it seems certain that the Lower Cambrian 

 must be conformable on the Proterozoic in areas which were continu- 

 ously under water (1) during the later part of the Proterozoic era, (2) 

 during the generally unknown interval between the Keweenawan and 

 Cambrian, and (3) during the Cambrian. Since it is improbable that 

 the larger part of the area which was receiving sediment during the 

 last period of the Proterozoic era became land at the end of that era, 

 it follows that areas where conformity exists between Cambrian and 

 Proterozoic are probably more extensive than areas where unconform- 

 ity exists. From a study of the relations suggested by Figs. 91 to 93 

 it is apparent that unconformity between Proterozoic and Cambrian 

 is likely to be most general just where their contacts are most frequently 

 seen. 



The Middle Cambrian. 



The strata of the middle division of the Cambrian (Acadian) are 

 known to occur in the same general localities as those of the lower, 

 both in the eastern and western parts of the continent, and in addition, 

 they occur in Texas, Oklahoma, Indian Territory, 1 Arizona, 2 and in 

 some parts of Montana, 3 and possibly even in the western part of Wis- 

 consin 4 where Lower Cambrian is absent, or at any rate not known. 

 This distribution indicates that the interior was being invaded by 

 the sea from the south and southwest before the close of the Middle 

 Cambrian epoch. Like the preceding series, the Middle Cambrian beds 

 are generally absent from the eastern interior. Where the Middle 

 Cambrian rests on the Lower, as in the east and west, the two series 



x Taff, Professional Paper No. 31, U. S. Geol. Surv. 



2 Blake, Am. Geol. Vol. XXVII, p. 162; Walcott, Bull. Geol. Soc. Am., Vol. X, 

 p. 217. 



3 Walcott, idem., p. 209. 



4 Walcott, idem., p. 224, 



