268 REPORT OF NATIONAL MUSEUM, 1892. 



II. SEDIMENTARY FORMATIONS, THEIR CHARACTER AND LIMITATION. 



It is apparent from statements made in the preceding essay that, as 

 a rule, to which there are no geologically important exceptions, fossils 

 have been preserved only in those accumulations of aqueous sediments 

 which are now known as the sedimentary or stratified rocks of the 

 earth, and that it is therefore these rocks only which furnish biological 

 data for geological investigation. Furthermore, it is upon the general 

 classification of these rocks, based upon both their physical and bio- 

 logical characteristics, that the whole system of historical geology 

 depends, and it is their fossil contents that furnish the most efficient 

 aid in the study of structural geology. The following essay w ill be 

 devoted to discussions of these subjects, and it is therefore desirable 

 first to present some remarks upon the stratified rocks with reference 

 to the origin ami limitation of the formations into which they are natu- 

 rally divided. These remarks will necessarily include both the physi- 

 cal and paleontological definition of formations, and a reference to 

 their use as units of stratigraphic classification. 



There has been much difference of custom among geologists as re- 

 gards the use of the term formation, some applying it to the smallest 

 assemblages of strata which possess common characteristics, while 

 others designate by the same term those series of formations to which 

 the term system has been generally applied. That is, some apply 

 the term formation to local or limited developments of strata, while 

 others apply it to such systems as the Devonian, Carboniferous, Cre- 

 taceous, etc. This term has generally been confined to the stratified 

 rocks, but by a few authors it has been applied to the eruptive, and 

 also to the great crystalline rock masses. In these essays, however, 

 the use of the term formation is not only confined to the stratified rocks, 

 but it is restricted to those assemblages of strata which have common 

 distinguishing characteristics, whether they have little or great geo- 

 graphical extent, or whether they aggregate a few feet or thousands of 

 feet in thickness. That is, the use of the term is herein confined to 

 those assemblages of stratified rocks of sedimentary origin* to which 

 many authors have applied the term group, and others the term ter- 

 rains 



A formation of this kind consists of an assemblage of strata which 

 bears evidence of having been deposited by continuous sedimentation 

 in a broad body of water, the sediments in different formations and 



* To avoid frequent repetition, the terms sedimeutary formation and stratified for- 

 mation are used interchangeably when applied to formations as defined in this essay. 

 The terms sedimentary rocks, stratified rocks, and fossiliferous rocks are also used 

 interchangeably, but with a somewhat more general meaning than is intended by 

 tin- two former terms. 



