RHYTHMS IN SEDIMENTATION 809 



"The Laurel limestone is usually hard, white, and evenly bedded. Toward 

 the south it becomes softer and more argillaceous. 



"The Waldron shale is a distinct bed overlying the Laurel, and is mainly a 

 clayey shale having the facies of the Osgood shale. It becomes calcareous 

 toward the north. 



"The Louisville limestone, which at Louisville is unconformably overlain by 

 Devonian limestone and has a thickness of 55 feet, is described as thinning 

 out toward the north and east. 



"The faunas of the several divisions of the Silurian in Indiana are distin- 

 guished by Foerste, who recognizes close affinities between those of the Laurel 

 and Louisville on the one hand and between those of the Waldron clay and 

 Osgood clay and limestone (below the Laurel limestone) on the other." ^* 



This section is seen to illustrate a series of disconformities of lesser 

 and greater time value. Without the testimony of the fossils there wonld 

 be no evidence of the great lapses of time unrepresented by sediments. It 

 is eloquent of the imperfection of the stratigraphic record and of the 

 lack of necessary relation between the thickness of a section and the time 

 represented, in that apparently conformable strata of three periods are 

 shown in one quarry face. 



Plate 4:6, figure 2, is a view of gypsum beds showing the rhythmic 

 crinkled banding in a chemical precipitate. Here the changing concen- 

 tration of the water may be due to changes in climate or to changes in the 

 freedom of access of sea-water to the lagoons in which precipitation pro- 

 ceeded. The well marked composite rhythm is best seen by looking at 

 the photograph from a distance or with nearly closed eyes, and the regu- 

 larity of this rhythm suggests that it is a climatic record rather than one 

 owing to the shifts in physical geography. Each lamina represents a 

 stage of warmth and concentrating water ; each surface of separation rep- 

 resents a shorter or longer period of undersaturation and consequent 

 absence of record. Gradations may represent continuous deposition, but 

 with changing rates. Such a series of beds is worthy of very careful 

 analytical study as a detailed meteorological record of the past. 



Part III.— Estimates of Time based on geologic Pkocesses 



MAGNITUDE OF THE CENOZOIC ERA ON EVIDENCE CHIEFLY FROM EROSION 



Time is measured by the recurrence of rhythms or by the aggregate 

 effects which are accumulated during their passage. The two preceding 

 parts of this article have dealt with the importance of rhythms in modi- 

 fying the rates of erosion and sedimentation ; yet it is seen that previous 



3* B. Willis : Index to the stratigrapliy of North America, Prof. Paper No. 71, U. S. 

 Geol. Survey. 1912, pp. 2.32, 2.33. 



