INTRODUCTION 55 



This naturally introduces us to a closer examination of geologic forma- 

 tions of various kinds. 



As sho^v^l above, while the geologic unit is the "formation," "life zones" 

 may occur in formations totally differing in thickness, in the kind of rock, 

 in geographic extent, in mode of tleposition. These largely geologic data 

 are, however, of constant service to the palaeontologist as part of the record 

 of the past conditions under which the animals lived. 



The map of North America on page 54 illustrates clearly the two 

 great divisions in the kinds of formations, namely, the border areas of 

 marine, estuarine, and fluvio-marine deposition of formations (indicated in 

 black) and the central areas of continental deposition (indicated in dots). 

 Connected with this distribution is naturally the power which transported 

 the sediments, whether of the sea or in inlets of the sea, whether of streams 

 or rivers, or even of the wind. Sorted as to the transporting power, the 

 various kinds of formations in which fossil remains of mammals occur are 

 as follows : 



1. Marine. Beneath the ocean or along its margins. Such formations occa- 

 sionally contain the remains of land and freshwater mammals, mingled with those 

 of marine mammals and shells. 2. Estuarine. Brackish water deposits along the 

 inlets of seas and at river mouths ; also indicated by the remains of animals. 3. Flu- 

 viatile. Freshwater sediments deposited in river channels or bays, at the mouths 

 of streams, or in torrent fans. 4. Lacustrine. Freshwater deposits borne into 

 lakes by rivers or streams, which beyond the coarser entrance areas may be of the 

 finest grain and become evenly stratified through periodic sedimentation. 5. Flood 

 plain or overflow deposits. Through periodic overflow, as of the Mississippi or the 

 Nile. Vast stretches of country flooded with muddy water, which subsiding may 

 also leave a stratified sedimentation. Very characteristic of the Middle Tertiary 

 of North America. 6. Lagoon deposits. In abandoned river channels and shaUow 

 lakes which through evaporation may collect gypsum and other salts. Very fre- 

 quent in the Caenozoic of France. 7. J^olian or aerial deposits. Transported by 

 the wind on dryland surfaces, usually fine non-stratified sands and dust, always 

 lacking the regular horizontal lines which may characterize lacustrine and flood- 

 plain deposits. Frequent in the later Caenozoic of North America. 8. Cave 

 deposits. Bones of animals living in caves, fallen or dragged into them, inclosed 

 with other fine sediments. Where over-abundant they may consolidate into a 

 "phosphorite " or phosphate deposit. Frequent in the Pleistocene of North America 

 and Europe. 9. Fissure deposits. Bones accumulated in the same manner as in 

 caves, or by wind or water action, in fissures of the rock, more or less consolidated, 

 also sometimes forming "phosphorites." Frequent in the Caenozoic of Europe, as 

 the famous fissure deposits of Egerkingen, of Lissieu, of Quercy. 



The remains of fossil mammals may be deposited under any of the 

 above conditions and thus occur in formations of many kinds. The least 

 perfectly preserved are those washed along with coarse pebbles and gravels, 

 while the most perfect as a rule are those found in the fine sediments of 

 still water, of aeolian dust, of asphaltum, or of volcanic ash. 



