332 REPORT OF NATIONAL MUSEUM, 1892. 



were deposited in marine, and others in nonmarine waters, and that 

 some of the latter deposits were formed in inland seas, some in lakes, 

 and some in estuaries or rivers, are necessarily inferential in character. 

 Still, the former existence of those previous conditions is held to be 

 demonstrated by means of certain accepted criteria. 



The only criteria of this kind which may be :*egarded as trustworthy 

 are based upon conditions which are observable with reference to now 

 existing oceanic and inland waters, and upon the character of the 

 organic forms which inhabit those waters respectively. They are there- 

 fore of two kinds, physical and biological. The physical criteria pertain 

 to conditions surrounding, or prevailing in the region adjacent to, each 

 of such bodies of water. The biological criteria pertain to the organic 

 forms for which any given body of water constitutes a congenial hab- 

 itat. As used in geological investigation, physical criteria are applied 

 mainly to the stratigraphical and lithological character of sedimentary 

 formations or deposits, to the method of their accumulation and to the 

 action of those natural forces which have characterized them or con- 

 trolled their production. Biological criteria are applied to the fossil 

 remains of animals and plants which the formations respectively are 

 found to contain. In some instances, however, the character of the 

 fossil remains is such as to imply the coexistence of certain physical 

 conditions which may not otherwise be plainly indicated. 



The distinguishing physical characteristics of fluviatile deposits, be- 

 sides the narrowness of their limits and the effects of self-corrasion 

 of their valleys, which are shown in terraces and bluffs, are the preva- 

 alence of shingle and sand in their channels, and of silt upon their 

 flood plains, and the absence of such regular stratification of any of 

 these materials as is to be observed in those which constitute marine 

 and lacustrine deposits. These characteristics are more or less plainly 

 apparent in the lew ancient fluviatile deposits which have been discov- 

 ered in association with formations which have been deposited in broad 

 bodies of water. It is true, however, that shingle and silt sometimes 

 accumulate in the marine waters of narrow straits or channels in such 

 a manner as to resemble fluviatile deposits, and that their wave-worn 

 bluffs often resemble some of those which have been produced by river 

 corrasion. While therefore it is sometimes practicable to recognize 

 among geological formations such fluviatile deposits as these by means 

 of physical data alone, it is always difficult and often impracticable to 

 do so, especially if their true character has become obscured by dis- 

 placement and erosion, or by the overlapping of other formations. 



These remarks are made with particular reference to those ancient 

 river channels which have been corraded out of sedimentary forma- 

 tions and covered by others of a similar kind, and not to those later 

 channels, some of which have become covered by glacial drift and 

 others by lava outflows. The earlier are usually less distinct and char- 

 acteristic than are some of t lie later ones, doubtless because the effects 



