INTRODUCTION AND SUMMARY 749 



Combining the indications regarding the present high rate of denuda- 

 tion with tlie evidence of the halting and disicontinuous nature of past 

 deposition, it is seen that geologic time is certainly much longer — perhaps 

 ten_g_r fifteen times longer — ^than the estimates based on strict]}- uiiiformi- 

 tarian interpretation. 



Judged in the light of these arguments, 250,000,000 years may be re- 

 garded as a moderate estimate, from the geologic data, of the lapse of 

 time since the beginning of the Paleozoic. The" indications from radio- 

 activity are in. fact that it is more thaii twice as long as even this figure 

 of 250,000,000 years, and the geologic evidence is seemingly not in con- 

 flict. An estimate of 250,000,000 is a return to the order of magnitude 

 of the guesses made by Lyell and Darwin before Sir William Thomson, 

 later Lord Kelvin, began to cut down to a small part of this estimate 

 the capital of years which could be drawn on by geologists from the 

 bank of time. liot recognizing the importance of the principles just 

 stated, and feeling the need, so far as possible, of meeting the demands 

 of the physicists, geologists found it easy to make lower estimates, though 

 difficulty was found in compressing them to the later narrow limitations 

 of 10,000,000 to 20,000,000 or 40,000,000 years for the age of the earth 

 as set by Tait, Kelvin, Eitter, King, and others. 



With tlie opening of the twentieth century the disicovery of radio- 

 activity disclosed, however, undreamed of stores of thermal energy. This 

 discovery revealed w4th startling suddenness the unfounded nature of 

 the assumptions, previously unquestioned, on which rested the restric- 

 tions that physicists had placed on geologists. Kot only did physicists 

 destroy the conclusions previously built by physicists, but, based on 

 radioactivity, methods were found of measuring the life of uranium 

 minerals and consequently of the roicks which envelop them. Instead of 

 limiting earth history to less than 40,000,000 years, they now granted 

 upwards of 1,500,000,000. Many geologists, adjusted to the previous 

 limitations, shook their heads in sorrow and indignation at the new 

 promulgations of this dictatorial hierarchy of exact scientists. In a way. 

 this skepticism of geologists w^as a (correct mental attitude. The exact 

 formulas of a mathematical science often conceal the uncertain founda- 

 tions of assumptions on which the reasoning rests and may give a false 

 appearance of precise demonstration to highly erroneous results. N'o 

 better illustration could Ije given than that of the case in point. This 

 skepticism was incorrect, however, unless it led to a careful and un- 

 prejudiced reexamination of tbe postulates on which rested the geologic 

 measurements of time. 



