Page:Popular Science Monthly Volume 76.djvu/602

598 plankton which can utilize the CO2 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 conclusion, 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 algæ 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 geologic 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 examples of such deposits may be cited the widely distributed Utica formation 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 belong to the nekton attached to floating seaweeds or other floating objects, 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 properties of animal oil and especially those of fish oil. 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 Sargossa Sea, which has among its algæ a very characteristic assemblage of animals. It is probable, however, that black shales having wide distribution were more often the deposits in closed arms of the sea (cul