598 THE POPULAR SCIENCE MONTHLY 



plankton which can utilize the C0 2 and the nitric acid compounds, 

 and so we see that upon these rest the greater part of the task of 

 elaborating the dissolved food-stuff of the sea" (239). 



Undoubtedly much of the land-derived nitrogen, estimated at 38 

 million tons per annum, is used up in the shallow areas by the plants. 

 TVe therefore arrive at the conclusion that shallow seas bordering 

 naked, cold, or arid lands should have the smallest amount of life, 

 and that those of temperate regions adjacent to low lands under pluvial 

 climates should have the greatest number of individuals. This con- 

 clusion, however, may be decidedly altered by the oceanic currents in 

 that they distribute far and wide the salts of the sea. 



These factors also suggest that during " critical periods " the 

 faunas should be least abundant and varied, and that at the times of 

 extreme base levels and sea transgressions they ought to be at their 

 maximum development. These suggestions are borne out by the small 

 Cambric, Permic and earliest Eocene faunas and the large cosmopolitan 

 biotas of the Siluric, Jurassic and Oligocene times. 



Sessile algas are not common on muddy or sandy grounds, and these 

 areas in the present seas have been compared with the desert areas of 

 the lands. That muddy grounds are now nearly devoid of algous 

 growth has particular significance in stratigraphy, because in the geo- 

 logic column at many levels and in nearly all regions occur black shale 

 formations that are not only devoid of plant fragments but are also 

 usually very poor in fossils of the sessile benthos. When the latter are 

 present it is seen that they are usually thin-shelled and small forms, 

 or are types of organisms that live in the upper sunlight realm and 

 are either of the swimming plankton or the floating nekton. As ex- 

 amples of such deposits may be cited the widely distributed Utica for- 

 mation of the Ordovicic extending from southern Ohio to Lake Huron 

 and east to Montreal, and the Genesee (Devonic) of New York. In 

 these cases what appears to be of the sessile benthos is thought to be- 

 long to the nekton attached to floating seaweeds or other floating ob- 

 jects, and eventually all of the life of the nekton and the plankton 

 sinks to the bottom of the sea. Therefore the carbonaceous matter of 

 the black shales may be of algous origin like that of the New York 

 Genesee, but it is far more probable that it is largely of animal origin, 

 as the crude petroleum of such deposits usually has the optical proper- 

 ties of animal oil and especially those of fish oil. 3 Plants may be torn 

 from rocky bottoms of the shallow areas by the action of the storms and 

 then carried by the currents into eddying areas like the present Sar- 

 gossa Sea, which has among its algge a very characteristic assemblage 

 of animals. It is probable, however, that black shales having wide dis- 

 tribution were more often the deposits in closed arms of the sea (cul 

 3 Dalton, Economic Geology, 1909, 627. 



