Page:The Journal of geology (IA journalofgeology11893univers).pdf/215

geologic time of 200,000,000 of years.^[1] Dr. James Croll estimated 72,000,000 years for the time duration since the first deposition of sedimentary rocks, while Sir Alfred R. Wallace thought that 28,000,000 years would suffice.^[2] Of the value of this estimate he says: "It is not of course supposed that the calculation here given makes any approach to accuracy, but it is believed that it does indicate the order of magnitude of the time required."^[3] Dr. Alexander Winchell reduced geologic time still more in his estimate of 3,000,000 years to the whole incrusted age of the world.^[4] Later writers, however, do not accept this, as we find Sir Archibald Geikie concluding on the basis of denudation and deposition that the sedimentary rocks would have required 73,000,000 of years for their deposition, if denudation was at the rate of one foot in 730 years; or of 680,000,000 of years if at the slower rate of one foot in 6,800 years.^[5] Mr. T. Mellard Reade adopted one foot in 3,000 years as the rate of average denudation throughout geologic time, and obtained a result of 95,000,000 of years as the time that had elapsed since the beginning of Cambrian time.^[6] M. A. de Lapparent is one of the few European continental geologists that has written on geologic time. On the basis of mechanical denudation and sedimentation he thinks that from 67,000,000 to 90,000,000 of years would suffice, at the present rate of sedimentation for everything that has been produced since the consolidation of the crust.^[7] The two most recent writers who have taken their initial datum point or "geochrone" from the consideration of late Cenozoic or Pleistocene phenomena