January 27, 1922] 



SCIENCE 



85 



derstauding of the many diatems, or interrup- 

 tions in the stratigTaphic column, as -well as 

 variation in the rate at which different types 

 of sediments have been laid down. 



Soon after the discovery of the fact that 

 radium undergoes an exceedingly slow decom- 

 position leaving helium and lead as products, 

 it was found that the minerals of the oldest 

 rocks contained more helium and lead than 

 those of younger rocks. The following are a 

 few examples selected from the calculations by 

 Strutt expressed in terms of the time from 

 certain peiiods down to the present : 



1. Miocene 6 million years 



2. Upper Carboniferous 146 million years 



3. Early pre-Cambrian 710 million years 



Holmes recognized that much of the gas 



emitted might have escaped during the subse- 

 quent existence of the minerals. Using the less 

 migratory element, lead, as a basis of calcula- 

 tion, he reached the figures 1,000-1,600 mil- 

 lions years for the age of certain minerals 

 from the Archean rocks. It is particularly 

 significant, that the testimony of different min- 

 erals from the same intrusion, considering the 

 errors expectable in analytic work, generally 

 agrees rather closely; and there is almost as 

 good a correspondence between the lead ratios 

 of minerals of the same general age but from 

 widely separated localities. According to Bar- 

 ren's interpretation, the rare-earth minerals of 

 the Llano uplift in central Texas contain 

 ui'anium-lead indicating an age of about 1,125 

 million years. They are roughly of middle 

 pre-Cambrian Age. Unless there are con- 

 cealed sources of error in the careful work of 

 our best radiologists, we seem justified in ac- 

 cepting their estimates as fairly satisfactory 

 approximations — very much the best we have 

 ever had — which indicate that the Mesozoic 

 rocks are tens of millions, the Paleozoic rocks 

 liiuidreds of millions, and the most ancient 

 pre-Cambrian rocks more than 1,500 millions 

 of years old. 



Reviewing these attempts to express the his- 

 tory of the earth in terms of years, we find 

 lliat from the time of Archbishop Ussher and 

 his estimate of 5,700 years for the total his- 

 tory of the earth up to his day, or even Lord 

 Kelvin who allowed 20 to 40 million vears for 



the entire evolution of the planet, the ten- 

 dency has been to greatly increase oui- esti- 

 mates of the length of time involved in even 

 the later part of the Earth's history; for we 

 must not forget that the time from the ear- 

 liest Archean period to the present is probably 

 very short, as compared with the time that has 

 elapsed in the formation of the earth as a 

 planet and is to elapse before its final refrig- 

 eration — if such is ever to occur. The contem- 

 plation of such vast lapses of time makes the 

 brief span of human history seem absurdly 

 short. It is diflicult to realize, when studying 

 ''history,'" commonly so-called, that we are 

 reading only the latest chapter of the last vol- 

 ume of a great series of tomes numerous 

 enough to fill a large library. 



The physical events that crowded this tre- 

 mendous space of years are being made known 

 as geologic field studies progress. Among 

 tlieni are the erogenic disturbances, or diastro- 

 phic revolutions, when long winding wrinkles 

 were bulged upon the Earth's surface, only to 

 ^\-aste away under the slow attack of the 

 weather. 'V^Tiile they are in this process of de- 

 oa,j, we call them mountain systems. At such 

 times, the shallow epicontinental seas are gen- 

 erally withdrawn into the ocean basins, the 

 average height of the continental masses above 

 sea-level is increased, and the work of erosion 

 accelerated. 



That part of North America now occupied 

 by the Rocky Mountain System was, from be- 

 fore the Cambrian down to the Cretaceous pe- 

 riod, for the most part a region of plains or 

 low hills, undergoing either general erosion or 

 decomposition. Much of the time it was sea- 

 bottom. Then followed a period of profound 

 disturbance. The stratified rocks were wrinkled 

 into folds, and their- foundation of ancient 

 crystalline rocks compressed and mashed be- 

 neath them. Great slices were sheared over 

 the subjacent mass for many miles. Molten 

 lava welled up into the disturbed formations 

 and even burst forth on the surface scattering 

 cinders and volcanic dust far and wide. Rem- 

 nants of the lava flows and stocks still mark 

 the sites of volcanic cones which have since 

 been demolished. This disturbance subsided, 

 but was repeated soon afterward at the close 



