22 PROCEEDINGS OF THE CHICAGO MEETING 



to consider that in many places wliere sediments of the finest texture are laid 

 down, there are animals and plants inhabiting the bottom of the sea, many of 

 which obtain their nourishment from organic material contained in the slime 

 and mud of the sea. Where deposition is slow, each annual layer, by the 

 action of these living organisms, will become mingled with other layers in a 

 homogeneous mass and can no more be measured separately than one scram- 

 bled egg can be separated from another. 



But it does not appear at all unlikely that in the past, as well as at the 

 present time, there are places where organisms only A'ery slightly, or not at 

 all, disturb the accumulating sediments. In glacial waters such undoubtedly 

 has been the case, and this is the reason why lamination in glacial clays re- 

 mains intact and trenchant. In most sedimentary rocks, especially in shales 

 and clays throughout the whole geological column, we find that lamination is 

 of frequent occurrence. We usually say that all sedimentary rocks are strati- 

 fied. Almost all of these rocks show seams which must represent interruptions 

 in deposition. These seams separate layers, ledges, or strata, which them- 

 selves represent time units of relatively uninterrupted deposition. The coarser 

 sediments, on the whole, consist of thicker layers than do the finer sediments. 

 Instances are not rare where we find considerable uniformity in the thickness 

 of layers for certain formations or parts of a formation. Thus we speak of 

 thin-bedded limestone and thick-bedded limestone; thin-bedded sandstone and 

 thick-bedded sandstone. I think we have all noted the uniformity in bedding 

 of certain limestones in the Pennsylvanian. The Carlinville limestone in Illi- 

 nois almost everywhere, under sufficient weathering, exhibits a remarkable 

 uniformity in this respect. It consists of layers measuring from two to four 

 inches. Such regular bedding occurs in the Canyon limestone in central Texas 

 and in the Permo-Carboniferous in west Texas. The separations between 

 these layers are so minute that they do not always appear in outcrops which 

 have not been considerably weathered. This bedding must represent some 

 rhythm. I believe it is to be accounted for by some seasonal or other cyclic 

 recurrence of a slight variation or interruption in the sedimentary process. 

 In one case I have observed, in a vertical section of about fifteen feet, in the 

 Comanchean sediments, a thi'ice-recurring cycle expressed in the repetition of 

 three different calcareous layers clearly representing dilferent conditions of 

 deposition. Here there appeared to be seen the results of three smaller cycles 

 within a larger cycle. 



In looking over the literature containing detailed sections of strata, one is 

 impressed with the fact that very few sections, as usually described, are sufli- 

 ciently minute in detail to be taken as a basis for the study of any smaller 

 cycles of sedimentation, such as may represent seasonal variations. The pub- 

 lished literatui'e contains very little that can be used for such studies as might 

 throw light on either an annual rhythm or on any rhythm of commensurately 

 longer cycles. Observations for the detection of any such phenomena must be 

 carried to the minutest detail possible. We have here a tabula rasa. Our 

 stratified rocks should be submitted to the minutest scrutiny for determining 

 'the smallest units into which their strata can be divided. Until a great mass 

 of observations of this sort can be compiled and analyzed, it would be unwise 

 to either affirm or deny the existence of evidence which may have a bearing 

 on the measurement of actual time. To me the attempt to collect such data 



