THE STRATIGRAPHIC COLUMN 383 



the Kinimswick of Missouri, with a combined thickness of at least 1,500 

 feet. This is followed by GOO feet of Trenton limestone, as shown in 

 central Pennsylvania and Tennessee, and finally by the Cincinnatian 

 limestones and shales, estimated at about 500 feet. 



The Silurian is probably underestimated rather than overestimated 

 at 3,000 feet. It may be all of 4,500 feet. The Richmondian, according 

 to present correlations, is not less than 1,000 feet, the most complete 

 section being on the Island of Anticosti. Counting the mainly clastic 

 deposits of the Appalachian Clinton formation as corresponding in 

 aggregate value to about 400 feet of limestone and the Waukesha, 

 Racine, and Guelph dolomites in east Wisconsin at 600 feet, the Niagaran 

 is estimated at about 1,000 feet. But according to the evidence brought 

 in by Kindle from the Porcupine River section in Alaska,^^ this amount 

 falls far short of the truth. Kindle, namely, estimates the Niagaran 

 dolomites in this region at about 2,500 feet. The Cayugan, judging 

 from the irregularly distributed deposits in central Pennsylvania, New 

 York, Ohio, and Michigan, seems to have an aggregate thickness repre- 

 senting something like 1,500 feet of limestone. 



The great thickness of 6,000 feet of Devonian limestone is found, 

 according to descriptions by Walcott and Hague, in the Eureka district 

 of Nevada. That this thickness is probably not too great for the whole 

 of Devonian time is suggested by the 1,200 feet of Helderbergian and 

 550 feet of Oriskanian limestone occurring, as described by J. M. Clarke, 

 in the Gaspe region of Canada. 



The accessible depositional records of the Waverlyan and Tennessean 

 systems are both thinner than any of the preceding systems. As used 

 in this paper, the Waverlyan comprises the beds beginning with the 

 Chattanooga shale and ending with the Keokuk. The Waverlyan seems 

 to be a time of considerable interruptions of sedimentation, and the 

 great difl'erence between the last of its faunas and the first of the Ten- 

 nessean argues for an especially long interruption at its close. This 

 point will be further discussed in Parts II and III. The maximum 

 aggregate thickness of accessible Waverlyan sediments, considered on 

 the basis of limestone values, is placed provisionally at 1,000 feet. It 

 may be that this figure is too high, but it is more likely to prove too 

 low. In the Mississippi Valley the succession of limestone deposits of 

 the Waverlyan system scarcely exceeds a total of 500 feet, but the evi- 

 dence of repeated interruptions of the process of deposition is convincingly 

 shown in both the Kinderhook and Osage series of limestones. Until 



^ E. M. Kindle : Journal of Geology, vol. 15, 1907, and Bull. Geological Society of 

 America, vol. 19, 1908. 



