THE STRATIGRAPHIC COLUMN 381 



thickness of "Carboniferous limestones" in the Cordilleran troughs seems 

 far too great for a single s^^stem. Indeed, when the deposits in t^e 

 several troughs have been properly correlated, I do not doubt that evidence 

 of oscillation and consequent general and local discontinuities of depo- 

 sition will be found that will sufficiently increase the already great 

 sedimentary record to fully warrant the three systems into which it is 

 proposed to subdivide it. 



THICKNESS OF ''PALEOZOIC" SEDIMENTS IN AMERICA 



So far as determined, the maximum thickness of marine deposits in 

 American continental seas from the beginning of Cambrian to the close 

 of the Pennsylvanian aggregates approximately 75,000 feet. Tliis total 

 includes deposits of all kinds, and therefore gives neither a true idea 

 of the time involved nor a satisfactory means of comparing the time 

 values of the successive systems. Manifestly, the rate of deposition is 

 widely different for the various kinds of sediments. That of the clastic 

 deposits, further, is exceedingly variable, being greatly influenced by 

 temporary and local conditions. The rate of limestone deposition prob- 

 ably also varied considerably according to its origin, its purity, and the 

 probable variability of conditions favoring chemical precipitation. Still, 

 it is the only kind of marine sediment for which we may reasonably 

 assume an average rate of deposition sufficiently uniform to warrant its 

 use as a basis in determining the respective time values of stratigraphic 

 units. Therefore, eliminating as much as possible the clastic deposits 

 and considering in their stead (1) the known calcareous sediments 

 regarded as corresponding in age to certain clastic sediments, and (2) 

 the probable limestone value of the sandstones and shales, of which no 

 e(|uivalent calcareous deposits are known (somewhat arbitrarily assumed 

 as 7 to 1), the total thickness of Paleozoic marine sediments corresponds 

 to something like 43,000 feet of limestone. This sum is divided among 

 the nine systems about as follows : Cambrian 8,000 feet, Ozarkian 6,500, 

 Canadian 4,200, Ordovician 5,700, Silurian 3,000 to 4,500, Devonian 

 6,000, Waver^yan 1,000, Tennessean 2,500, Pennsylvanian (including 

 lower Permian) 4,600. 



The great thickness of 8,000 feet or more of Cambrian limestone is 

 computed from Cordilleran sections recently published by Walcott^^. 

 As given by this authority the total thickness of the Cambrian in Utah 

 is not far from 19,000 feet. The lower 12,000 feet consist almost 

 entirely of quartzites and siliceous shales. Considerable limestone is 



*» C. D. Walcott : Cambrian geology and paleontology, No. 5. Smiths. Miscell. Coll., 

 vol. liii, 1908, pp. 167-230. 



