THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 19 



carry such Igneous fragments from far north or even farther west, if the 

 samples were taken below the mouth of the Missouri River, or the fragments 

 might have been gathered from the glacial drift of the banks. Certainly 

 the content of a fossil river-channel would present no less difficulty in the 

 interpretation of its origin. However, in such deposits the banks are 

 usually observable and much possible error easily avoided. 



d. Fossil content. — The life of marine and even fresh waters may to some 

 extent show the nature of the adjacent land as the habits and food-supply 

 .cause it to multiply or disappear upon shores of different character. 



Reefs of Bryozoa and corals give a direct suggestion. The animals are 

 fixed in position; the tentacles and mouths are directed upwards; any 

 large amount of sediment falling through the water would speedily cause 

 the extinction of most forms, though some are found living in very muddy 

 waters. The occurrence of reef corals in large numbers in any place suggests 

 clear water far from the mouths of great rivers or muddy currents, and 

 opposed to coasts from which relatively little material is being washed out — 

 a land with low or wooded slopes upon which the solvent processes of 

 degradation are more active than those of mechanical disintegration. 



Other animals, as many mollusks, live near muddy coasts, or on sandy 

 flats, etc. The student must here turn to some treatise on zoology for a 

 discussion of the life habits of various organisms. (See also Walther, 

 Einleitung in die Geologic, II Theil, and Grabau, A., Principles of Stratig- 

 raphy, chapter 28, Bionomic Characteristics of Plants and Animals.) 



IV. THE FOSSIL CONTENT OF THE UNIT. 



(a) The Fauna of the Unit. 



The fossils of animals are apt to be found in all kinds of deposits ; marine 

 beds carry by far the largest number, but terrestrial beds are frequently 

 rich in the remains of animal life. All fossils found in water-laid beds are 

 not aquatic forms; the remains of purely land animals find their greatest 

 chance of preservation when swept into bodies of water, or buried in the 

 deltas, sand-bars, mud-flats, etc., of rivers. In the case of floating carcasses, 

 distended by the gases of decomposition or supported by floating vegetation, 

 the remains might be swept far beyond the limits of deltas in large bodies 

 of water and come to rest in the horizontal beds of quiet, deep water. 

 Agassiz found remains of land vegetation on the bottom of the sea in the 

 Antilles, 1,000 fathoms down, and the Challenger dredged up plant remains 

 from as much as 1,400 fathoms in Polynesia.-^ Animal remains might go 

 approximately as far from the lands and sink in as deep water. 



Land birds and insects are frequently blown far out to sea, only to perish 

 and sink to the bottom or be devoured by fishes. Such accidental inclusions 



1 Suess, E., The Face of the Earth, English edition, vol. 11, p. 248. 



