EXTENT OF PALEOZOIC CONTINENTAL SEAS 315 



Among the more notable and commonly cited examples of cycles of 

 deposition is (1) that beginning with the Cambrian quartzites and end- 

 ing with the Ordovician limestones and shales, (2) beginning with the 

 Medina and ending with the Niagaran limestones and Salina mixed de- 

 posits, and (3) beginning with the Oriskany and ending with the Middle 

 Devonian Onondaga limestone and Hamilton shale. Taking them at 

 their face value, it must be admitted that these instances are not alto- 

 gether satisfactory from the theoretical standpoint. The cycles are cer- 

 tainly not coordinate, and none fits a major unit of the stratigraphic 

 column accurately. The petrologic sequence, too, is not the same in 

 different areas, nor are the correlated beds always even approximately 

 contemporaneous. But if the contemporaneity of the deposits concerned 

 in the cycle is not to be considered, if the feature of coordinateness may 

 be disregarded, and especially if the continuity of the process of sedi- 

 mentation covered by the cycle is not a factor of vital importance, then 

 these are good examples. But what useful purpose could such cycles 

 serve in working out a systematic history of earth diastrophism ? In- 

 deed, when carefully analyzed according to present knowledge of stratig- 

 raphy, these and other great cycles fail so badly that we may well doubt 

 the possibility of using the prevailing conception of cycles of deposition 

 in framing a satisfactory scheme of stratigraphic classification. Judi- 

 ciously modified, the principle may attain considerable taxonomic value. 



Coordinated and classified, the cycles would probably assist materially 

 in fixing the boundaries of many stratigraphic units. They would help 

 also in the elucidation of the diastrophic history of the continents. They 

 might be arranged somewhat as Willis has attempted to do (op. cit., page 

 247) into "grand cycles" corresponding to the eras of the time scale and 

 into several lower grades of "epicycles." But the cycles are too uncertain 

 in development, distribution, and possibilities of interpretation to enable 

 us, solely on the basis of their physical manifestations, to discriminate 

 truly between the cycles of major and minor import. Other means of 

 correlation are essential. Apparently their classification is possible only 

 in the light of the fossil organic record. 



PALEOZOIC CYCLES 



For any one to cite Cambro-Ordovician sedimentation as 'a single cycle 

 of deposition, which is comparable with the early Silurian and middle 

 Devonian cycles, is to expose great ignorance of the actual sequence of 

 Eopaleozoic deposits. The lower Cambrian usually begins with rather 

 coarse clastic deposits. In the Appalachian Valley these are sometimes 

 followed by shaly middle and upper Cambrian beds, and these again bv 



