294 PAST CLIMATES AND CLIMAXES. 



Volcanic or extrusive processes may produce stases in several ways. The 

 most important of these are found in the "petrified forests" of Arizona, 

 Yellowstone Park, and elsewhere. The latter are Miocene forests which were 

 buried by volcanic ejecta. The tnmks became petrified, and the mass of 

 volcanic material was then later removed. Such stases, though mere climax 

 fragments, leave nothing to be desired in the matter of permanence. Volcanic 

 dust may also bury forests and other vegetation, much as dunes do, the dead 

 trunks finally reappearing and persisting for a long time. Volcanic bombs 

 and lava flows often set fire to woodland, and the latter may then persist for 

 some time as a burn stase, often leaving a final trace iu a charcoal line or layer 

 in the soil. Volcanic gases may also destroy vegetation and produce a tem- 

 porary stase, and a similar effect is caused by the steam and hot water or mud 

 from geysers. Bum stases are most frequently produced by man, and they 

 constitute a record of the first importance in the study of secondary succes- 

 sion (Clements, 1910). Many of man's other activities, as well as those of 

 animals, produce fragmentary and temporary stases of some importance, 

 particularly in connection with various natural agencies. The most striking 

 examples of this are found in the culture horizons of early historic and pre- 

 historic periods. In the case of lake dwellings, and paleolithic and neolithic 

 caves, hiunan and aqueous agencies have cooperated to leave a record of the 

 greatest value, though the botanical part is the least satisfactory. 



Water stases. — ^As abeady indicated, water is by far the most important 

 agency in the production of stases. This is due to the fact that water alone 

 permits the development of one serai stage at the same time that it is accom- 

 plishing the preservation of the preceding one. Hence it alone can produce 

 stases in which the serai sequence is completely preserved. All waters have 

 this property in some degree, but it is especially characteristic of peat or acid 

 bogs, and hence peat stases are the most perfect of all, though the closely 

 rdated coal stases approach them in this respect. In addition to these com- 

 plete stases, water also produces incomplete or fragmentary ones. This is 

 of course typically the case when the water is shallowed and the water-content 

 reduced in the course of succession. Such an outcome is the final fate of every 

 peat-bog, and marks the limit of each imit stase. In the case of swamps 

 proper, and of many ponds also, the stase consists only of the hydrophytic 

 layers, from algae to sedges, or it may show but one or two of these. In such 

 cases, the algal or charad layer consists largely or wholly of inorganic material, 

 i. e., of the lime incrustations of the filaments, while the succeeding layers 

 consist of the remains of water plants and amphibious plants respectively. 

 A similar basal inorganic layer of sUica, often of much thickness, is found in 

 swamps, such as those of Yellowstone Park, which have been derived from 

 lakes teeming with diatoms. Deposits of diatoms during more remote periods 

 are indicated by the vast beds found in Nebraska, California, and elsewhere 

 (plates 55, a, b; 51 a). 



Water likewise produces fragmentary stases of the greatest interest. Such 

 are the travertine deposits of lakes, and the travertine and suiter deposits 

 of hot springs and geysers. These are related to marl and diatom earth in 

 that they are due to plant activity, and consist of limestone or siHca. They 

 differ in being deposited as rock which must be weathered and eroded before 

 the algal pioneers can be followed by other consocies, and hence they do not 



