DEFORMATION AND GRADATION. 297 



the period shows subdivisions or epochs. Thus the system of formations 

 deposited during the Cambrian period is divided into three series, the Lower, 

 Middle, and Upper Cambrian, corresponding to three epochs, viz, the Earlier, 

 Middle, and Later Cambrian. 



As might be expected, there is no essential correspondence between the 

 stratigraphic units as determined by geology and those based upon vegeta- 

 tion. The eostrate has no equivalent, though it is clearly the whole sequence 

 of systems for a vegetation era. The clistrate may be larger or smaller than 

 a system, while the cUstase is always smaller. Costrates in some cases corre- 

 spond to formations, while costases do so only exceptionally, and are for the 

 most part the equivalents of beds. Similarly, strates and stases may corre- 

 spond to beds, but they are most frequently to be regarded as lamina, it would 

 appear, while the associal layers of a peat stase would constitute still smaller 

 divisions. 



Belatlon to succession. — It is sufficiently obvious that the developmental 

 study of past vegetations must rest primarily upon the plant record. The 

 major features of development will be recorded in eostrates, cUstrates, and 

 clistases; the minor ones in costrates and costases, and in simple strates and 

 stases. These may often correspond to geological divisions and formations, 

 but usually they do not. In either event, the plant evidence must be para- 

 mount for the ecologist, and reasons have already been given to show that this 

 may prove true for the paleozoologist and geologist as well. When the 

 geological and zoological evidence agrees with the botanical, the value of the 

 latter is enhanced. When they are in conflict, the ecologist can but accept 

 the plant evidence as primary, and then consider the conflicts in relation to 

 the sequence of cause and effect. As has been indicated above, the present 

 limits and sequence of the geological periods may harmonize sufficiently well 

 with major developmental facts in vegetation. At any rate, they can well be 

 used until a lack of harmony becomes convincing and confusing. 



DEFORMATION AND GRADATION. 



Processes. — ^From the standpoint of succession, all forces which act upon 

 the Uthosphere either increase or decrease the area on which vegetation can 

 develop. With reference to the development of the Uthosphere itself, all such 

 forces belong to one or the other of two great processes, viz, deformation and 

 gradation. Deformation has to do with all crustal movements, and especially 

 those which affect the relations of land and water. Gradation includes all the 

 processes concerned in molding the surface by the transport of material; it 

 consists of the complementary parts, erosion and deposition. The removal 

 of matter from the land by any agent is degradation; aggradation, its deposi- 

 tion upon the land or in the sea. The normal result is to tend to reduce the 

 earth's surface to a common level. The temporary effect, however, may often 

 be to increase the relief of the surface, as, for example, in the formation of 

 dimes along sandy shores. While vulcanism is usually regarded as distinct 

 from deformation and gradation, it is often associated with movements of 

 the crust, on the one hand, while the transportation of lava and ash has much 

 in common with gradation. Thus the two basic processes of crustal develop- 

 ment, deformation and gradation, in the widest senses, are essentially corre- 

 lated and antagonistic. The one tends in general to increase the irregularities 



