THE PLANT RECORD. 



295 



occur in the serai sequence. The travertine terraces of Mammoth Hot 

 Springs do show a sequence, but this is a sequence of deposit and not of 

 successional development, since the algse are the same in each. Naturally, 

 when the terraces are once weathered into soil, a primary sere begins to 

 develop, but this has no organic relation to the algal stage. Alluvial soils 

 frequently bury vegetation, and thus give rise to fragmentary stases, and per- 

 colating waters may have the same effect in caverns. Running water highly 

 charged with mineral matter may form deposits, such as calcareous tufa, in 

 which plant parts are caught and preserved. 



Kinds of stases. — ^The unit stase or stase proper is the series of layers which 

 results from a single sere. It is best seen in a peat-bog, where the development 

 has continued uninterruptedly from open water to the climax, which is usually 

 forest. The layers lie in the sequence of development, and correspond in 

 number to the associes, though the earUer layers of aquatic plants are often 

 poorly developed. In the case of swamps not invaded by bog-forming mosses, 

 the stase is incomplete, consisting only of the earUer stages of the sere. Where 

 water is not the agency concerned, the stase is fragmentary, containing 

 usually a single layer. A costase consists of two or more stases; it is essen- 

 tially the record of a cosere. Hence its component stases are not necessarily 

 complete, since denudation or disturbance may initiate a new primary or 

 secondary sere at any time. A clistase is one in which the climax layer of 

 each stase differs from that of the preceding or following stase. It is a record 

 of the seres which result when the climax is changed by a swing of climate, as 

 in a glacial period. The peat-bogs of glaciated regions furnish the best exam- 

 ples, though even here bogs may be so recent as to show only a stase or a 

 costase. Clistases are typical of Scandinavia and Britain in particular, for 

 the bogs of these countries show the changes of climatic climaxes in remark- 

 able fashion. Costases and chstases occur in all the three vegetation eras, 

 but they are subordinate to the eostrate in all. Even when stases were most 

 characteristic, as in the Pennsylvanian, they occupy but a small part of the 

 system, approximately but 1 to 5 per cent of the total thickness. Conse- 

 quently the correspondence between strates and stases ends with the cHstase. 

 There is no eostase which characterizes each era as a complete series. There 

 are only costases and clistases which occur repeatedly in the eostrate, and are 

 characteristic but not predominant in the Pennsylvanian, Cretaceous, and 

 Quaternary systems. 



Eelations of strate and stase. — Typically, the strate and stase are wholly 

 distinct structures. Exceptionally, the one may pass into the other. Where 

 they pcciu-, such transitions are best seen just below or above a stase. Sedi- 

 mentary material is constantly being laid down in lakes and swamps. In 

 proportion to the organic material, the inorganic detritus is most abundant at 

 the beginning of the stase. Hence there is often a gradual transition from the 

 silt or clay bed upon which a stase rests to the layers of nearly pure peat above. 

 The intermediate layers of peat derived from the aquatic plants are often very 

 heavily charged with detritus, and are correspondingly valueless for fuel. 

 Similar conditions are found in coal-beds. There is usually a layer of clay 

 beneath each coal-bed, and this often grades through black shale into usable 

 coal, particularly at the margin, where the inwash was chiefly deposited. This 

 transition usually occurs also in the upper part of each bed, the accumulation 



