292 PAST CLIMATES AND CLIMAXES. 



The strate. — ^A strate is a more or less definite fossil horizon. It may con- 

 tain one or more layers due to changes in the rate or manner of deposition, but 

 the vegetational unity of the layers is indicated hy the similarity of the popu- 

 lation, i. e., by the floristic. It consists essentially of inorganic materials, 

 though the actual number of fossils embedded in it may be very great. It is 

 characterized primarily by its inorganic composition, and by the scattering 

 and mixing of the included fossils. 



While strates may be formed in consequence of deposit by various agencies, 

 some of the latter, such as wind, volcanoes, and man, exert this effect but 

 rarely or exceptionally. The conditions requisite to fossiUzation occur regu- 

 larly only in water, and hence strates arise normally only ia bodies of water. 

 The aerial movement of the plant parts concerned must have had an important 

 r61e in making fossilization possible, but the decisive part must have been 

 played by the water-body, especially with reference to its nature and size, and 

 the varying conditions of deposit. In order to appear in a stase, a species 

 must have been carried in whole or in part to a water-body. It must have 

 withstood destruction or decomposition in the latter, until it became embedded 

 in layers where the conditions were more or less favorable to fossilization. 

 The selective action of wind and water, and of the conditions of decay and 

 preservation must have been so extreme that only the smallest fraction of 

 plant hfe can have become a part of the record. In quantity, this part must 

 have been infinitesimal; in quaUty it affords a fioral picture of constantly 

 increasing completeness. In fact, it does not seem improbable that we shall 

 some day have at our command much the larger part of the floristic material 

 of the various eoseres. 



From its origin, the strate bears no fixed relation to development. This is 

 clear, since its fossils are brought by chance from many sources. As a rule, 

 however, it is characterized by the species of climax communities. For this 

 there are many reasons. The climax formation, once estabhshed, must have 

 occupied nearly the whole of its area at a time when disturbances by man were 

 absent. Moreover, bodies of water in which fossils could be formed most 

 readily, such as lakes and mature rivers, must have been bordered chiefly by 

 forest and scrub. While the leaves of woody plants were doubtless more 

 readily fossilized than those of herbs, the strildng paucity of fossil grasses in 

 particular must be explained otherwise. The plausible explanation of this 

 is that grasses occupied the drier highlands back from lakes and sluggish 

 streams, and were seldom carried into the latter by wind or water. More 

 important still, grass leaves do not fall from the parent plant. They not only 

 remain dried upon it, but the plants also remain firmly fixed in the soil as a 

 rule. Moreover, the incoming of grasses must have resulted in a marked 

 development of herbivores, with the consequence that grazing played a deci- 

 sive part in reducing the nmnber of grass parts available for fossiUzation. 

 Further light seems to be thrown upon this problem by the behavior of the 

 Bermettitales during the Mesophytic era. This group probaby represented 

 the scrub and grass life-forms of dry areas and regions during this era, though 

 they doubtless occurred as undergrowth in the coniferous forests as well. 

 Their position relative to water-bodies seems clearly indicated by the fact 

 that enormous numbers of leaves have been fossiUzed, but stems and other 

 plant parts are of the rarest occurrence, except in a few favored localities. The 



