ESTIMATES OF TIME 819 



"The area of the Cordilieran sea is given as 400,000 square miles, but it was 

 probably 600,000, if uot much more. It may be claimed that the area tributary 

 to the Cordilleraii sea was greater than I have estimated. The evidence, such 

 as it is, is against such a view. As a whole, I think the estimate of 17,500,000 

 years for the duration of Paleozoic time in the Cordilieran area is below the 

 minimum rather than above it." ^^ 



In discussing Walcotf s results it should be noted first tlia't Eeade's 

 estimate of 70 tons of calcium salts as the denudation rate per square 

 mile per year for the globe consists of 50 tons calcium carbonate and 20 

 tons calcirun sulphate. Clarke, on the basis of much better data, con- 

 cludes that these figures are almost twice too large.^"* Instead of choos- 

 ing a larger rate of supply to the ocean than the present, it would seem 

 that it would be better to assume it as far lower. In Part I of this 

 article reasons have been advanced for holding that the average relief of 

 the Paleozoic was very low and the land areas suffering erosion were 

 restricted. On this basis Walcott's results might be multiplied by five 

 or ten. 



As to the rate of deposition, Walcott makes no allowance for intervals 

 of non-deposition. At the time he wrote it was generally believed that 

 the Paleozoic seas reigned continuously throughout that era, but with 

 somewhat progressively restricted areas. He was in advance of his time 

 in showing that in the Lower Cambrian the continent was largely 

 emerged, but there was not as yet any knowledge of the many diaeon- 

 formities which represent shorter and longer times of no record. The 

 discontinuity of these Paleozoic water bodies is indicated on Schuchert's 

 paleogeographic maps of successive epochs of the Paleozoic. A total of 

 36 maps is given for the Paleozoic. On five Scliuichert shows no Cor- 

 dilieran sea whatever. On 11 others it has but slight representation, 

 the region of more permanent water being in Nevada and connecting 

 with the Pacific or southern Atlantic waters. 



It is clear that a higher rate of limestone deposition will take place 

 in shallow, warm, and agitated waters; but that is only possible so long 

 as the water is deep enough to prevent bottom-scour. Walcott chose a 

 high rate of deposition of lime for a limited area over the globe, of which 

 he reckoned the Cordilieran sea to be four-tenths, whereas it might per- 

 haps equally well have been taken as not more than two-tenths of the 

 area of the epeiric seas. The actual rate of deposition as controlled by 

 the amount of subsidence was something less than this possible rate. 



esLoc. cit, pp. 672, 67.3. 



58 F. W. Clai-ke : A preliminary study of chemical denudation. Smithsonian Misc. 

 Coll., vol. .50, no. 5, 1910, p. .5. 



