50 GEOLOGICAL BIOLOGY. 



least 48,000,000 of yearj, or Geikie's of from 100,000,000 to 

 680,000,000. We find at one extreme the ancient theory of 

 6000 years, and at the other McGee's possible maximum of 

 7,000,000,000 years. The rate of accumulation of sediment 

 over the bottom of the sea may vary between the limits of 

 one foot in 730 years and one foot in 6800 years, as pointed 

 out by Geikie, the figures being based upon the estimated 

 proportion between the annual discharge of sediment in cubic 

 feet and the area of river basins in square miles, in the case 

 of the rivers Po and Danube. The estimate of 680,000,000 

 of years, quoted above, is dependent upon the assumption 

 that the total thickness (maximum) for the sedimentary de- 

 posits is not less than 100,000 feet, and that the average rate 

 of accumulation was not more rapid than that now going on 

 at the mouth of the Danube, based upon Bischof's determina- 

 tion of the amount of sediment and matter in solution in the 

 Danube at Vienna. It may be a query worth considering 

 whether the estimates based upon the examination of the 

 amount of suspended and dissolved matter in river water are 

 not likely to err in the direction of too small amount of mat- 

 ter by reason of the abnormal precipitation along the course 

 of the river incident to the presence of salts and acids put into 

 the river by man. If the rate of the river Po were taken, the 

 length of time would be 73,000,000 of years instead of 

 680,000,000, 



The actual length of time in years, however, is of less 

 importance to the geologist than the relative length of time 

 for each of the eras, and these latter, the time-ratios of Dana, 

 are deducible from the physical thickness, and size of constit- 

 uent particles, of sedimentary rocks. Relative thickness is 

 certainly one of the elements in the determination of the time- 

 values of the geological formations, and the fields for investi- 

 gation, along which greater accuracy is to be reached, include 

 the problems of the rate of accumulation of muds, sands, and 

 pebble beds, and of the formation of limestones, in relation 

 to each other and under varying conditions, and the detection 

 of the marks in the strata recording the conditions incident 

 to the varying rates of accumulation. In making estimates 

 of time, as represented by thickness of deposits, there should 



