DEFORMATION AND GRADATION. 305 



The materials for this have been drawn chiefly from Chamberlin and Salis- 

 bury, and the numbers refer to pages in volumes 2 and 3 of their "Geology." 

 It is evident that not all of these cycles are of the same intensity and duration. 

 Indeed, this could hardly have been true of any two of them. In the present 

 state of our knowledge, however, they do stand out as members of the major 

 sequence marked by similar great climatic cycles and vegetation changes. 



It seems probable that the last three or four cycles, as well as the periods, 

 loom larger because of their nearness to the present. Their actual corre- 

 spondence may well be with the secondary cycles, which produced epochs in 

 certain of the Paleozoic periods. Since the Phocene deformation was of the 

 first rank, rivaling the great pre-Cambrian movement, the other recent cycles 

 may well be regarded as more or less equivalent to most of the cycles of the 

 series. Moreover, all of the larger cycles contained secondary ones of sufli- 

 cient intensity to suggest that they may belong to the major series. Another 

 source of diflSculty lies in the fact that deformation or gradation often began 

 before the close of one period and extended well into the other, producing a 

 transition epoch, such as occurs between the Ordovician and Silurian, Triassic 

 and Jurassic, Jurassic and Comanchean, Cretaceous-Eocene, etc. Finally, 

 the Permian-Jurassic cycle transgresses the limits of the Paleozoic, just as the 

 Eocene cycle began in the upper Cretaceous (c/. Chamberlin and Salisbury 

 2:639; 3:38, 3:162). All these discrepancies in correspondence indicate 

 the difliculty if not the impossibiHty of assigning definite limits to eras or 

 periods, owing to the fact that the complete deformation sequence of gradation, 

 climate, vegetation, and fauna must often or always have extended over two 

 or more periods. 



Deformation and unconformity. — ^It is clear that a regular consequence of 

 deformation and the associated sea withdrawal is the exposure of the beds 

 laid down in the previous gradation phase, to erosive action during the new 

 gradation phase. The following submergence by sea-invasion, subsidence, or 

 both brought about the deposit of new beds upon the eroded surface, producing 

 unconformity. Such a series of events must have been characteristic of mar- 

 ginal areas of sedimentation, since they must have shown a recurring alterna- 

 tion of emergence and submergence, of erosion and sedimentation for every 

 general body deformation: 



"Where the Cambrian is unconformable on pre-Cambrian formations, there 

 is a break in the geological record. Such breaks are sometimes said to 'repre- 

 sent 'lost' intervals, the intervals which are 'lost' being the periods elapsing 

 between the deposition of the beds below the unconformity, and those above. 

 This designation for such an interval is only partly true, for in the uncon- 

 formity itself there is the record of exposure and erosion, followed by sub- 

 mergence and deposition. The general sequence of events is evident and to 

 this extent the record is not lost. On the other hand, the products of the 

 recorded erosion were deposited elsewhere, constituting a new formation " 

 (Chamberlin and Sahsbury, 1906 : 2 : 222.) 



Obviously, conformity indicates continuous or renewed deposition in a 

 water area; unconformity, alternating land and water stages, with correspond- 

 mg erosion and deposit. Just as the latter are the complementary parts of 

 the complete process, so unconformity and conformity always have a necessary 

 developmental connection. A section through an unconformity shows two 



