GEOLOGICAL TIME. 591 



tain the probable rate of progress, we may approximate to the 

 length of time it required. Hall and Lyell estimated the average 

 rate at one foot a year, — which is certainly large. Mr. Desor con- 

 cluded, after his study of the falls, that it was " more nearly three 

 feet a century than three feet a year." Taking the rate at one 

 foot a year, the six miles will have required over 31,000 years ; if at 

 one inch a year, — which is 8+ feet a century, — 380,000 years. These 

 calculations may be taken as data for estimating the length of time 

 required for excavating the great gorge of the Colorado mentioned 

 on p. 569, — 300 miles long and 3000 to 6000 feet deep, some hun- 

 dreds of feet of the depth being for much of the distance through 

 granite. The whole was probably accomplished after the close of 

 the Mesozoic. 



The rate at which coral reefs increase in height affords another 

 mode of measuring the past. The rate of growth of the common 

 branching Madrepore is not over one and a half inches a year. 

 As the branches are open, this would not be equivalent to more 

 than half an inch in height of solid coral for the whole surface 

 covered by the Madrepore, and, as they are also porous, to not over 

 three-eighths of an inch of solid limestone. But a coral plantation 

 has large bare patches without corals ; and the coral sands are 

 widely distributed by currents, part of them to depths over one 

 hundred feet, where there are no living corals ; not more than one- 

 sixth of the surface of a reef-region is in fact covered with growing 

 species : this reduces the three-eighths to one-sixteenth. Shells and 

 other organic relics may contribute one-fourth as much as corals. 

 At the outside, the average upward increase of the whole reef- 

 ground per year would not exceed one-eighth of an inch. 



Now, some reefs are at least 2000 feet thick, which, at one-eighth 

 of an inch a year, corresponds to 192,000 years. If the progressing 

 subsidence essential to the increasing thickness were slower than 

 the most rapid rate at which the upward progress might take place, 

 the time would be proportionally longer. 



The use of these numbers is simply to prove the proposition that 

 Time is long, — very long, — even when the earth was hastening on 

 towards its last age. And what, then, of the series of ages that lie 

 back of this in time? 



In calculations of elapsed time from the thickness of formations, there is 

 always great uncertainty arising from the dependence of this thickness on a 

 progressing subsidence. In the case of coral limestone the data employed give 

 the least possible time, as is obvious from the above. In estimates made from 

 alluvial deposits, when the data are based on the thickness of the accumulations 

 in a given number of years, — say the last 2000 years, — this source of doubt affects 



