RELATION OF BIOLOGY TO GEOLOGICAL INVESTIGATION. 28] 



needing definition, the latter being applied to all faunas that are re 

 garded as having lived in either brackish or fresh waters. 



In making practical application of the evidence which has been re- 

 ferred to, it is the general conditions which are indicated by the special 

 that will most need to, be considered. For example, the marine char 

 acter of a fauna having been ascertained, the conclusion is legitimate, 

 in the absence of conflicting evidence, that the area which constituted 

 its habitat was bordered wholly or mainly by other marine areas, and 

 that their sediments and faunas blended to a greater or less extent with 

 one another. In other words, such a fauna indicates that its habitat 

 was part of a great oceanic expanse which was occupied by other 

 more or less similar faunas. 



In case a fossil aquatic fauna should present intrinsic evidence of its 

 brackish water origin the inference would be legitimate that its habitat 

 was either an estuary or an inland sea, and in case the fauna should 

 prove to be of fresh water origin we must conclude that the habitat 

 Avas either a river or a lake. In all of these nonmarine cases the habitat 

 had more definite boundaries than could have been the case with that 

 of any marine fauna, and usually, but not necessarily always, a non- 

 marine formation has a less geographical extent than have marine for- 

 mations. It is true that an estuary fauna blends in part with the 

 adjacent marine fauna on the one hand and with the tluviatile fauna on 

 the other, but its other limits are shore lines such as alone constitute 

 the faunal boundaries of all other nonmarine bodies of water. 



The estuarine, tluviatile, or lacustrine origin of a deposit or formation 

 having been ascertained by means of the character of its fossil remains, 

 aided by the accompanying physical indications, important inferences 

 are to be drawn as to the geographical conditions which prevailed in 

 that region at the time of its deposition. For example, the existence 

 of an estuary deposit implies that contemporaneously with its deposi- 

 tion there was an adjacent body of marine water, and also a large land, 

 if not a continental, area which was drained by the inflowing river.* 

 Again, every lake or inland sea, the former existence of which may be 

 determined by the character of the fossil aquatic fauna which the for- 

 mation representing it contains, was necessarily surrounded by a broad 

 land area. 



The foregoing remarks apply to methods of distinguishing between 

 formations of marine and nonmarine origin, and to the legitimate 

 inferences that may be drawn from them, respectively, as to the physical 

 conditions which prevailed while they were accumulating. In closing 

 this essay it is desirable to present some remarks upon the relative 

 value in practical geological field work of the fossils found in marine 

 and nonmarine formations, respectively. 



That the fossil remains of marine faunas are far more valuable as in- 



' There are, of course, estuaries at the mouths of those rivers which flow iuto lakes, 

 but brackish water estuaries only arc here referred to 



