ESTIMATES OP TIME 821 



est weight to the age of the earth as 80,000,000 years, based on the 

 sodium content of the sea, he grants from 23,000,000 to 29,000,000 years 

 for unconformities. By assuming Precambrian time to have been as 

 long as later time, his total estimate from sedimentation is raised to the 

 desired total of 80,000,000 years. ^^ According to these estimates, the 

 Paleozoic era was about 25,000,000 years in duration. 



In discussing Sollas" results, the first point to be noted is the rate of 

 denudation chosen — one foot in 2,400 years. This is more than three 

 times as rapid as the present mean for the whole earth determined by 

 latest measurements, and more than twice as rapid as tlie late of denuda- 

 tion for the Mississippi drainage basin, which, except for tlie (-olorado, 

 is given by Dole and Stabler as the highest of the twelve large drainage 

 divisions of the United States. In defense of choosing such a liigh rate, 

 it may be said that the maximum thicknesses in geosynclines have come 

 especially from bordering uplands or mountains, but on the otlier hand 

 these mountains were rapidly reduced in slope, long periods of quiet and 

 low relief intervened between uplifts, and the average slope for the Paleo- 

 zoic was certainly but a fraction, of the present high reliefs. Instead of 

 taking a mean rate of denudation as one foot in 2,400 years, the present 

 mean rate might better have been taken, or the present rate might have 

 been even divided by two, giving a denudation rate of one foot in 8,000 

 to 16,000. years — limits somewhat less than one-third to one-sixth of that 

 which he used. 



Sollas speaks of the coarseness of the geosynclinal deposits as proof of 

 rapid deposition; but these coarse beds are only occasional, and the great 

 bulk of the deposits are such as would have been carried by streams of 

 gentle grade. 



Turn next to his fundamental assumption of one to ten as the ^atio 

 of the deposition area to the denudation area. Instead of making care- 

 ful estimates from many streams and comparing these to the geographic 

 relations implied by the geosynclines, the Gulf of Mexico diainagc is 

 chosen offhand as a sufficient basis. It is doubtful if a poorer illustra- 

 tion could have been found ; as is seen when it is noted that now, in a 

 time of abnormal continental relief, the Mississippi and other streams 

 gather together the waters and the waste from the broad interior and 

 high mountainous tracts of a continent and converge it radially into a 

 Mediterranean sea, the Gulf of Mexico, where the deposition is mostly on 

 the slopes of a continental terrace. 



The thick formations of the past are not deposits of the continental 



Quart. Tour. Geol. Soc, vol. 6.5, 1909, pp. cxiii-cxv. 



