THE DIVISIONS OF THE GEOLOGICAL TIME-SCALE. 59 



time-duration expressed. The maximum thickness of the 

 whole series is estimated to be about 100,000 feet, or 20 

 miles.* 



Samuel Houghton estimated that the time represented by 

 the intervals between the strata, when deposition was not 

 going on at the locality where the strata are examined, was as 

 great as that recorded by them. This will fully make up for 

 the error from overrating the maximum thickness. Measur- 

 ing the greatest thickness recorded on the earth for each of 

 the various formations, Houghton estimated the aggregate to 

 be 177,200 feet. 



Upham proposes to increase this figure to account for 

 undiscovered strata, and places the total maximum thickness 

 of stratified rocks at 50 miles, or 264,000 feet. Thus, re- 

 garding thickness, we have estimates ranging from 100,000 

 to 264,000 feet. It may here be stated that the average thick- 

 ness of the total known strata of the world is somewhere 

 near 80,000 feet. 



(2) Another element entering into the question of rate of 

 accumulation of deposits is the rate of removal of mineral 

 substances carried from the continents into the ocean in solu- 

 tion (see Dana, Geikie, and others). 



Mr. Reade estimates that the soluble minerals removed 

 from England and Wales in this way, mainly Calcium and 

 Magnesium Carbonates and Sulphates, would equal I foot 

 removed from the whole surface of the area in 12,978 years. 

 Prestwich estimated I foot in 13,000 years for the area of 

 drainage of the Thames, and for the world an average of 100 

 tons per square mile annually, with an assumption that the 

 amount removed mechanically is six times as great, or total 

 (600 -|- 100 =) 700 tons per square mile. 



Houghton, f adopting the estimate of the rates of denuda- 

 tion of river-basins required to lower the entire rain-basin a 

 thickness of one foot to be as follows : 



* See Walcott, " Geologic Time, as indicated by the Sedimentary Rocks of 

 North America": Proc. A. A. A. S., vol. XLII. 1893, pp. 129-169. 

 f See Nature, vol. xvin. 1878, pp. 266-268. 



