270 REPORT OF NATIONAL MUSEUM, 1892. 



Because of the frequent displacements of portions of the eartli crust 

 which have occurred during geological time, and also because of the 

 resulting great erosion of the materials of which formations are com- 

 posed, none of them now exists in its original entirety, but it is desirable 

 to consider them with reference to their origin as well as to their pres- 

 ent condition, the better to understand their character. 



Every formation originally consisted of sediments, which, within a 

 portion of geological time of limited but uncertain duration, were de- 

 posited in any broad body of water, whether inland or marine, during 

 all of which time both the body of water and the surrounding or ad- 

 jacent land remained comparatively unaffected by displacements of the 

 earth's crust or by any other adverse physical changes. Under favor- 

 able conditions every such body of water was the congenial habitat of 

 aquatic animals, remains of which became fossilized in the constantly 

 accumulating sediments. These animals constituted more or less dis- 

 tinct faunas, the geographical range of each of which was, in the case 

 of inland waters, throughout the whole area occupied by each, and in 

 the case of oceanic waters, throughout each of such large portions as 

 were circumscribed by natural intramarine limits to fauna! distribu- 

 tion, these limits having been in a general way barriers between 

 faunal areas, although they were never sharply defined and were 

 usually indefinite. 



Within these more or less indefinite intramarine barriers the various 

 forms of aquatic life constituted a separately recognizable fauna, and 

 the sedimentary deposits became a separately recognizable formation. 

 Still, those barriers are properly designated as indefinite because it is 

 evident that the sedimentary deposits of any one of those ancient 

 oceanic areas always blended to a greater or less degree with those of 

 adjacent areas, and that certain members of every fauna ranged into 

 adjacent faunal areas, just as certain species of every living marine 

 fauna have a much wider geographical range than have most of their 

 associates. 



Much the greater part of the sedimentary formations of the earth 

 were deposited in marine waters, as is shown by the character of 

 their contained fossils, and most of those waters were then, as now, of 

 oceanic extent. The character of the contained fossils of a compara- 

 tively small but important part of the sedimentary formations, how- 

 ever, show that they were deposited in inland bodies of water, some of 

 which were fresh and some brackish. Some of those inland bodies of 

 water were comparatively small, but others were of such extent that 

 their deposits rival marine formations in that respect. 



Every formation resulting from deposits in iidand waters hav- 

 ing had practically the same geographical extent as the body of water 

 in which it was deposited, the original boundary of the whole forma- 

 tion was coincident with the shore line, but this can not be assumed 

 with regard to the formations which were deposited in waters of 



