EVALUATION OF STRiVTIGRAPHlC CRTTERTA 425 



color rhythm is remarkably regular, but in other ])laces numerous fine, 

 delicate, and even hairlike green bands are distributed at very irregular 

 intervals throughout red beds. Everywhere the individual bands are 

 strikingly persistent. 



At one point a number of dolomite beds were noted to occur inter- 

 calated in the banded mudstone series, and throughout a thickness of 50 

 feet the formation consists of alternating slates and dolomites, the dolo- 

 mite bands ranging in thickness from one-fourth of an inch to about 2 

 feet, and the slate bands from one-eighth of an inch to several feet in 

 thickness. 



RELATIONS OF BEDDING TO MODE OF SEDIMENTATION 



Method of presentation. — The preferred method of science is induc- 

 tion, the accumulation of such a variety of observations, covering all the 

 possible cases of occurrence, that from their classification tlie hiws which 

 control the phenomena may be determined; but induction is unsafe if 

 based on partial observations. Where the principles which control the 

 operations of nature are better known than their results, deductive rea- 

 soning, on the contrary, is the safer guide to conclusions, but is a method 

 which needs to be checked as much as possible by appeals to observation. 



For a study and comparison of the characteristics of bedding as con- 

 trasted in the deposits of the subaerial and subaqueous plain, the induc- 

 tive method calls for observations to be made on modern sediments of 

 tlie two regions now in process of deposition, and on ancient sediments, 

 whose conditions of origin are known from otlier criteria. Such observa- 

 tions are, however, as yet so iiisufhcient in variety and usually so qualita- 

 tive in character as to be unsafe when used alone for the inductive deter- 

 mination of distinctive criteria. For example, cross-bedding is known to 

 occur in both fluviatile and marine deposits and no convincing distinc- 

 tions have been drawn between the two by an observer who has fully 

 studied both. This section of the subject, therefore, can best be treated 

 deductively, drawing the distinctions which should result from the prin- 

 cij^les which control wave and current action and checking the conclu- 

 sions as far as possible by citation of tlie known facts of stratigrai)hy. 



Lannnation of mudstones — Effects of subsidence from suspension. — 

 Lake or estuarine clays, if deposited below the deptli affected by waves 

 and currents, are characterized by a very regular laminjvtion which is 

 commonly closely spaced and may give rise to paper shales. The ma- 

 terials are wholly derived from suspension in water and are not marked 

 by the intercalation of sand lenses. The same is true of marine mud- 

 stones, but tlie more powerful waves of the open seas are able to affect a 

 greater depth of water and restrict to such depths the areas free from 



XXXI— Bull. Geol. Soc. Am., Vol. 23, 1911 



