304 PAST CLIMATES AND CLIMAXES. 



3 : 192). Their definition, which states that "the time involved in the reduc- 

 tion of a land area to base-level is a qjcle of erosion," likewise implies that an 

 erosion cycle is limited to a particular area. A similar view appears to be 

 held by Cowles (1911 : 181), as indicated by the statements that "within one 

 climatic cycle there may be many cycles of erosion," and "each erosive cycle 

 within the climatic cycle in turn has its vegetative cycle." Since minor as 

 well as major deformations produce corresponding climatic cycles, the erosion 

 cycles of Cowles seem to be definitely limited both in time and in space, and 

 to be quite distinct from the grand deformation-gradation cycles which pro- 

 duced the primary climatic changes. 



Period or era. Dejormaiion cycle. 



Proterozoic [Deformation, 196, 218. 



Cambrian < Gradation (and sea invasion), 238, 267. 



Ordovician [Gradation, 304. 



Silurian [Deformation, 332, 336, 368, 395. 



Devonian < Gradation, 395, 418. 



Mississippian (Gradation, 496, 499. 



Pennsylvanian fDeformation 507-510. 



(Gradation, 496. 



Permian [Deformation, 619, 639, 3: 48. 



Triassic Reformation, 3: 48, 38, 60. 



Jurassic (Gradation, 38, 60, 79. 



Comanchean fDeformation, 67-69, 106, 124. 



Cretaceous \Gradation, 106-107, 137. 



Eocene fDeformation, 161-163, 162, 194. 



(Lower Oligocene) .... (Gradation, 194. 



Oligocene fDeformation, 195. 



Miocene (Gradation, 195. 



Pliocene fDeformation, 196. 



Pleistocene {Deformation, 196. 



Human (Gradation, 518. 



The series of deformation cycles. — ^We have already seen that deformation 

 and gradation alternate as cause and effect. Deformation renews or increases 

 gradation, and the latter plays a large or controlling part in the production 

 of stresses which initiate a new deformation. This primary relation is the 

 obvious explanation of the complete periodicity of deformation cycles. Every 

 general body deformation is followed by a great period of relative quiescence 

 marked by gradation and sea invasion, but it is this very period which causes 

 or at least makes possible the accumulation of stresses that results in new 

 deformation (Chamberlin and Salisbury, 1906:2:657). If this assumption 

 of the mutually causal relation of deformation and gradation be correct, then 

 the geological record should furnish evidence of a complete series of cycles 

 from the earliest times to the present. A scrutiny of the record shows that 

 this is the case. Beginning with the Proterozoic era, body deformation and 

 cosmic gradation have followed each other regularly and inevitably to the 

 final great deformation of the Pliocene and the relatively quiescent gradation 

 period of the present. This fundamental sequence of deformation cycles 

 and its correspondence with geological periods is shown in the table above. 



