212 W. Upham — Estimates of Geologic Time. 



within 30 miles from the coast, and all the coast lines of the 

 earth have a total measured length, according to Dr. James 

 Croll and Mr. Wallace, of about 100,000 miles ; so that the 

 deposition is almost wholly confined to an area of about 

 3,000,000 square miles. This area is one nineteenth as large 

 as the earth's total land area ; hence it will receive sediment 

 nineteen times as fast as the land is denuded, or at the rate of 

 about nineteen feet of stratified beds in 3,000 years, which 

 would give one foot in 158 years. With this Wallace com- 

 pares the total maxima of all the sedimentary rocks of the 

 series of geologic epochs, measured in whatever part of the 

 earth they are found to have their greatest development. 

 Prof. Samuel Haughton estimates their aggregate to be 177,200 

 feet, which multiplied by 158 gives approximately 28,000,000 

 years as the time required for the deposition of the rock strata 

 in the various districts where they are thickest and have most 

 fully escaped erosion and redeposition. 



Most readers, following this argument, would infer that it 

 must give too large rather than too scanty an estimate of 

 geologic duration ; but to many students of the earth's strati- 

 graphy it seems more probably deficient than excessive. All 

 must confess that the argument rests upon many indeterminate 

 premises, since the total extent of the land areas and the 

 depths of the oceans have probably been increasing through 

 the geologic areas, and the effects of tides have probably 

 diminished. The imperfection of the geologic record, so 

 impressively shown by Charles Darwin in respect to the 

 sequence of plants and animals found fossil in the rocks, will 

 also be appealed to as opposing the assumption that the 

 177,200 feet, or 33^- miles, of strata represent the whole, or 

 indeed any more than a small fraction of the earth's history. 

 To myself, however, this last objection seems unfounded, 

 since in many extensive and clearly conformable sections ob- 

 served on a grand scale in crossing broad areas, there is seen to 

 have been evidently continuous deposition during several or 

 many successive geologic epochs, and by combining such sec- 

 tions from different regions a record of sedimentation is made 

 wellnigh complete from the earliest Paleozoic morning of life 

 to its present high noon. But perhaps we may do better to 

 change somewhat the premises of our computation, in view of 

 the extensive regions where the rock strata remain yet to be 

 thoroughly explored, and because of certain large land tracts 

 having little rain and therefore no drainage into the sea. Let 

 us assume that the total maxima of strata amount to 50 miles, 

 and that the mean rate of the land denudation is only one 

 foot in 6,000 years ; and we then obtain a result three times 

 greater than before, or about 84,000,000 years for the deposi- 

 tion of the stratified rocks. 



