RiiYTaMS liT SEDIMENTATION 807 



nature of the rhythms^ but it is regarded as very important that a rhythm 

 runs with minor changes a certain distance through the strata and then 

 comes to a full stop. It would be expected that, in a complete record, 

 a rhythm should gradually emerge and disappear. The differences be- 

 tween A, B, and C are expressive, then, of well marked diastems between 

 them. Lesser diastems divide the beds which make A, B, and C. Between 

 C and D'is a diastem of such large value that when sedimentation was 

 resumed in D the nature of the rhythm had entirely changed. If this 

 series is interpreted in accordance v^ith the harmonic rhythms shown in 

 figure 5, it is seen that a composite oscillation of baselevel with alternate 

 fill and scour would account for the observed features of tlie beds. Eaeli 

 of the finest laminae may have been deposited in a year, but the whole 

 may represent only a fifth, tenth, or smaller fraction of the entire time. 

 A long time interval is implied by the diastem between C and D, probably 

 longer than the time represented by the whole of the Lowville formation, 

 though not great enough to produce a notable fauna! change on the 

 resumption of the stratigraphic record. 



The next illustration, plate 45, figure 1, showing also a change in 

 rhythms, is from the Maysville or Lorraine group, which is the middle 

 group of the Upper Ordovician or Cincinnatian. The Lorraine sea, ac- 

 cording to Schuchert, was a somewhat limited flood covering most of the 

 area between the Mississippi, the Appalachian land on the east, and the 

 region of the Great Lakes. Winchell and Ulrich make the following 

 statement regarding the section in Ohio : 



"At the base of the (Lorraine) division, which at Cincinnati comprises about 

 200 feet of strata, there are some arenaceous layers that on weathering fre- 

 quently preserve the fossils as casts. Above these there are numerous layers 

 of crystalline limestone, 3 to 10 inches in thickness, separated by relatively 

 thin bands of shale. In the upper 60 or 70 feet the bedding is more irregular 

 and the limestone layers thinner and generally argillaceous, unfitting them for 

 building purposes. Fossils are well preserved and exceedingly plentiful and 

 among them may be recognized nearly every species that has been described 

 from the equivalent beds in New York." '^ 



All the stratigraphic features indicate shallow water. The crystalline 

 limestones suggest alternating undersaturation and super saturation of the 

 water, with alternate solution and precipitation of what was originally 

 organic debris. The abundant fossils suggest periodic rapid burial in 

 wave-stirred muds. A long period of time is represented by a very limited 

 depth of strata. In fact, on a diastrophic basis, the Cincinnatian may 



33 B. Willis : Index to the stratigraphy of North America. Prof. Paper No. 71, U. S. 

 Geol. Survey, 1912, p. 166. 



