SOIL FORMATION. 



83 



sive diatom beds in various places in Nebraska, California, Nevada, etc. 

 While the production of diatomaceous soil may be seen along the margins of 

 many pools and streams, diatom marshes of large extent are rare. Weed 

 (1887) has traced their development in Yellowstone Park, and has found that 

 extensive meadows have been built up in this way. In spite of the difference of 

 material and the absence of certain secondary influences, the primary reaction 

 has to do with the decrease ia the amount of water as ia the case of peat areas. 



(2) Reaction by accumulating plant concretions. — The rocky substrata due 

 to the direct physiological activity of plants are either calcareous or siliceous, 

 the former being much more common. Calcareous substrata are represented 

 by marl, travertine, calcareous tufa and perhaps by oohte; siliceous ones by 

 sinter or geyserite. Concretions of either sort are usually formed by algae 

 and are especially characteristic of hot springs. Aquatic mosses also possess 

 the power of secreting travertine and tufa. Chara plays the chief rdle in the 

 formation of marl (Davis, 1900, 1901), while Rothpletz (1892) assumes that 

 oohte is due to the calcareous secretions of a blue-green alga. Cohn (1862) 

 was the first to point out the connection of algae with the formation of tufa 

 and sinter. The first studies of importance in this country were made by Weed 

 (1889) in Yellowstone Park, and these have been supplemented by those of 

 Tilden (1897, 1898) in the Rocky Mountain region generally. Tilden has 

 described 24 algse from the hot springs of this region, and it is probable that 

 all of these play a part in rock formation. The yellow-green algae (Chloro- 

 phyceae) are represented by Oedogonium, Hormiscia, Conferva, Microspora, 

 Rhizoclonium, and Protococcus. The blue-green thermal algae (Cyanophyceae) 

 belong to the genera Calothrix, Rivularia, Hapalosiphon, Schizothrix, Symploca, 

 Phormidium, Oscillatoria, Spirulina, Synechococcus, Gloeocapsa, and Chroocoo- 

 cus. In the case of the marl or lime deposit of lakes, Davis finds that it is 

 made up of coarser and finer material derived from the incrustations on 

 Schizothrix and Chara, but principally the latter. 



From the standpoint of succession, concretion into solid rock is very differ- 

 ent from that by which marl is produced. The compactness of travertine, 

 siater, and oolite is doubtless due to the microscopical size of the algae con- 

 cerned. In the case of marl formed largely by Chara, the stem and leaves of 

 the latter are so large relatively that their death and decay breaks up the con- 

 cretions in large degree. The fragile branching stems and leaves also prevent 

 compacting into a soUd mass. Marl, moreover, accumulates in ponds and 

 lakes, where its action is to shallow the water and to produce much the same 

 results already noted for peat and diatom soils. In fact, the action is essen- 

 tially identical so far as the initiation of the water sere and the direction of the 

 first stages are concerned. Sinter and travertine are formed locally as super- 

 ficial deposits under conditions which are mrfavorable to colonization, though 

 this does begin at the edges of the cooler brooks which drain the hot-spring 

 areas. The essential fact, however, is that they are biogenic rocks and can 

 only form initial areas for primary succession, instead of directing the sequence 

 of stages. As in the case of tufa and oolite, the reaction of the concretionary 

 algae leads to the origin of a new rock sere, while in the formation of marl by 

 Chara it continues and directs a water sere already begun (plates 4, 51 a, 58 b). 



(3) Reaction by producing weathering. — The primary reaction of plants 

 upon rocks is the decomposition of the surface into an exceedingly fine soil. 



