468 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



unconformities be included in the evidence on which a period of world- 

 wide deformation is postulated, since these criteria may have been con- 

 fined to marginal areas now submerged or to such as were subsequently 

 baseleveled and later covered by more recent deposits. Such criteria may 

 indeed be found in certain areas now accessible to investigators, but being 

 absent elsewhere they are commonly assumed to be manifestations of 

 merely local movement and correspondingly unimportant taxonomically.' 



PRE-PALEOZOIC PERIODS OF DIASTROPHIC ACTIVITY INDICATED BY 

 GRADATION AL CRITERIA 



Paleozoic periods of diastrophic activity thought to be of the highest 

 rank are indicated by data which, though incomplete, may be interpreted 

 satisfactorily. The late pre-Gambrian period of activity is generally 

 suggested by the unconformable relations of the Cambrian to the under- 

 lying crystallines on which it usually rests. But as a rule we know little 

 concerning the time value of the hiatus at the base of the Cambrian. 

 The Proterozoic covers a very long time, and we are seldom able to decide 

 positively just which part of it, if any, is represented by tlie rocks be- 

 neath the Cambrian. In most cases these rocks are much older tban the 

 Belt series which, in the Great Basin, underlies the lower Cambrian with 

 only moderately angular unconformity. Lithologically, the Belt series 

 is not greatly different from the lower Cambrian, so that in areas where 

 the two are in contact a grand period of diastrophism and attendant 

 erosion is not clearly suggested. Obviously then, while the postulated 

 pre-Cambrian period of great activity is undeniable, the determination of 

 the actual time relations of the movements is based on rather indefinite 

 data. Nevertheless, I am convinced that the lower Cambrian was laid 

 down during the closing stages of a grand period of diastrophic activity 

 and surface gradation. This conviction rests principally on the fact that 

 gentle folding of the Belt series, which lies in the negative Cordilleran 

 basin, occurred before the Cambrian, and on the probability that the 

 major folding and erosion of the time took place on the more positive 

 areas of the continent. At the same* time it seems highly probable that 

 the folding and consequent erosion of the Proterozoic rocks, other than 

 the Belt series, belongs in part to much earlier periods. 



LATE ORDOriCIAN-EARLY SILURIAN PERIOD OF ACTIVITY 



Tt is on the same kind of evidence — the invariably clastic character of 

 the deposits and their great thickness in areas commonly given over to 

 limestone deposition during the Paleozoic — on which tbe early Cam- 

 brian period of diastrophism is based, that a similar period of activity is 



