152 DYNAISIICAL GEOLOGY. 



pelagic life. (See further, page 143.) The stratified limestones and other 

 rocks of North America have no true deep-water characteristics. Wyville 

 Thomson gave this as his general conclusion for all continents. 



3. Deposits made by Continental Species. 



1. SILICEOUS DEPOSITS. 



Conferva-like Algae, having columnar, vase-shaped and furze-like forms, 

 grow in the hot geyser waters of the Yellowstone Park, which secrete opal- 

 silica freely throughout the plant, as first reported by W. H. Weed. They 

 cause the deposition of the silica from the waters in a gelatinous form, 

 making the geyserite basins and the wide-spread geyserite deposits. These 

 siliceous plants are described as growing an inch upward in 10 weeks. 



Diatoms make beds in shallow ponds over the contineiits, and thick 

 deposits of them are common beneath the peat of ordinary marshes. Such, 

 ponds have only the gentlest of waves ; but sufficient to break into pieces 

 most of the infinitesimal shells. 



Diatoms are especially abundant in the warm waters of the Yellowstone 

 Park, where the beds made from them cover many square miles in the 

 vicinity of active and extinct hot springs, and vary from three to six feet 

 in depth. Near Monterey, Cal., there is a Diatom bed 50 feet thick. Others 

 occur in Nevada, where, according to Ring, they alternate with beds of tufa ; 

 and some are 200 or 300 feet thick. The material of the beds looks like 

 chalk, but it often becomes partially solidified to opal, of a brown, yellowish, 

 or greenish color. 



2. CALCAREOUS DEPOSITS. 



1. The shells of terrestrial and freshwater Mollusks are mostly thin and 

 fragile, especially the Gastropods, breaking easily under the gentlest wave 

 action. Limestones with unbroken shells as fossils are of rare occurrence 

 and small extent, forming only in bodies of water too shallow for wind- 

 waves. The more common genera are Sphmrium, Limnma, Pliysa, Planorhis, 

 Paludina, and Papa. The deposits over the bottoms of small ponds are 

 usually accumulations of pulverized shells, and have a chalky aspect. The 

 earthy and clayey beds of river valleys ordinarily contain nothing of the 

 shells of the valley except minute grains from their wear, or calcareous 

 concretions made from the grains. The fine earthy loess of large valleys 

 is remarkable for the number of its freshwater shells (Gastropods), its 

 strongly calcareous character, and its calcareous concretions, and bears 

 evidence thus of the sublacustrine and shallow conditions attending its 

 deposition. 



2. Loosely textured calcareous rock, called tufa because of its appearance, 

 is formed from the confervoid Algse of the Yellowstone Park and other 

 regions. It is an aggregation of the algoid growths, some of which resemble 

 somewhat the concretionary forms represented in Fig. 137 on page 132. 



