LESSONS IN OEOLOOY. 373 



often asked, but seldom answered with any degree of satisfaction. 

 The matter of geologic time bears on so many important ques- 

 tions that much time has been given to the investigation of all 

 available data on tl^ subject, and many estimates have been 

 made. The early estimates in general were large. Sir Charles 

 Lyell considered that the earth was at least 240,000,000 of years 

 old. Dr. Charles Darwin thought that 200,000,000 of years 

 could hardly be sufficient for the evolution of organic forms. Dr. 

 A. Winchell, 1883, estimates the time since the crust was formed 

 at 3,000,000 years. Mr. W. J. McGee, 1892, estimated the age 

 of the earth at 15,000,000, and Prof. Warren Upham, 1893. 

 thinks the time necessary for building up the stratified rocks and 

 developing the plant and animal life of the world must be about 

 100,000,000 of years. These results would seem to indicate that 

 the data were not satisfactory, or else that they were not cor- 

 rectly interpreted. Mr. Charles D. Waleott, Aug. 17, 1893, in an 

 elaborate paper reviews the whole question, working out an esti- 

 mate that seems conservative when compared with those given 

 by other investigators. Confining his investigations mainly to 

 the cordilleran region of North America, he shows that erosion 

 and deposition of mechanical and chemical sediments have al- 

 ways been going on at substantially the same rate. While one 

 foot in 3,000 years is usually considered an average rate of denu- 

 dation, he assumes a rate of one foot in 200 years for the region 

 under consideration. Then, carefully estimating the area of ero- 

 sion, the area of deposition, and the thickness of sediments, both 

 mechanical and chemical, he estimates the cenozoic, including gla- 

 cial or pleistocene, at 2,900,000 years; mesozoic at 7,240,000 

 years; paleozoic at 17,500,000 years; algonkian at 17,500,000 

 years and archsean at 10,000,000 years, a total of 55,140,000 

 years. He concludes that the time since the archaean age lies be- 

 tween 25,000,000 or 30,000,000 of years as a minimum and 

 60,000,000 or 70,000,000 as a maximum time. 



The physicist, calculating on the time necessary for the earth 

 to cool to its present condition, limits the time to from 10,- 

 000,000 to 30,000,000 of years. These estimates may not be very 



