322 STUDIES FOR STUDENTS 



become more fully appreciated, it is nevertheless true that the greater 

 part of the sediments comes to rest beneath the sea. Yet the fact 

 that in the past the possibility of other modes of origin has not been 

 sufficiently held in mind, throws doubt, in the opinion of some geol- 

 ogists, upon the interpretation of certain ancient formations consist- 

 ing of unfossiliferous sandstones and shales. 



Further, in regard to those deposits which are universally conceded 

 to be of fresh- water origin, it has formerly been unconsciously assumed, 

 without adequate proof, that the deposits were laid down in perma- 

 nent bodi3S of standing water, and hence were lacustrine rather than 

 fluviatile or ceolian. To illustrate the important consequences flowing 

 from such an interpretation, it may be mentioned that in all of the 

 older American literature it was always confidently stated, without 

 discussion of other possibilities, that the Tertiary was characterized 

 in the Rocky Mountain region and over the Great Plains by enormous 

 fresh-water lakes, larger than any in existence in the world today. 

 Within a few years this view has been vigorously combated, and 

 Matthew, W. D. Johnson, Haworth, Davis, and Hatcher have shown 

 that the greater number of the Tertiary formations are better inter- 

 preted as the deposits of aggrading rivers wandering over broad 

 flood-plains. 



In view of the several modes of origin which are possible for shales, 

 sandstones, and conglomerates, and the variable interpretations which 

 have been sometimes given for a single formation, the writer, in that part 

 of a lecture course upon advanced structural and dynamical geology 

 which deals with sedimentary structures and their origins, has been 

 accustomed for the past two years fif'st, to discuss the conditions of 

 formation of sedimentary deposits, as they are observed to occur at 

 the present day in various continents and under various climatic con- 

 ditions; second, to compare the relative areal-and volumetric importance 

 of the several kinds of deposits forming at the present time; third, 

 to discuss the probable changes in the relative importance which may 

 be expected to have occurred in the earlier ages, owing to such general 

 movements of continental uplift and subsidence, mountain making, 

 climatic change, etc., as are generally recognized to have charac- 

 terized and individualized the preceding ages; fourth, to give the 

 detailed distinctions in composition, texture, and structure by which 



