656 



SCIENCE 



[Vol. LV, No. 1433 



evident in the history of plant life. Life eame 

 from the sea; it emerged from the surface of the 

 salt waters, but the plant life of the sea which 

 first migrated from the waters to the earliest 

 continents of the earth was of a high order of 

 seaweed or algal growth, that is to say they were 

 algae which had developed strong permanent tissue 

 and special organs. In the view of modern stud- 

 ents of paleobotany these so-called "algae of 

 transmigration ' ' were of a higher specialization 

 than any algae now existing in the sea. The 

 time when they got their footing on the land is 

 indicated by the fact that in the ancient Preeam- 

 brian rocks which make the basement or founda- 

 tion upon which all later rocks have been laid 

 down, there is positive evidence that at different 

 periods in their own history these rocks were ex- 

 posed to the air and suffered weathering and so 

 produced a soil, the evidence of which could not 

 have been preserved to this day except through 

 the agency of a contemporary vegetal covering, 

 so that the time of the emergence of plant life 

 of a high order from the sea to the land is far 

 back in the dawn of the time records of the rocks ; 

 and the duration of time required for their de- 

 velopment longer even than is indicated for the 

 animals. As all estimates of concrete expressions 

 of time for the age of the earth based on bio- 

 logical data are bound to fail, the comparative 

 expressions given herewith must serve to intimate 

 a time duration for the organic history of the 

 earth so vast as to be beyond the possibility of 

 human expression. 



Age of tlie earth from the astronomical view- 

 point: Ernest W. Brown. 



The age of the earth: William Duane. In 

 estimating the age of the earth one should choose 

 as a clock to measure the time that has elapsed 

 some process in nature that takes place in one 

 direction only, and that does not change its rate 

 when conditions (temperature, pressure, etc.) 

 alter. In most of the estimates of geological 

 periods of time that have been made the clocks 

 employed do not fulfill these conditions. Esti- 

 mates based on the temperature of the earth or 

 sun, for instance, cannot be reliable, for the 

 temperature of a body may rise or it may fall, 

 and, further, the rate of its change depends upon 

 a variety of conditions, such as the amount of 

 radiation, the supply of energy to it, etc. In 

 the study of radioactivity during the last twenty- 

 five years a large number of transformations of 

 one chemical element into another have been 

 found. Students of the subject agree that these 



transformations take place in one direction only, 

 i. e., from an element of higher atomic weight to 

 an element of lower atomic weight. Further, no- 

 body has been able to alter the rate of a radioac- 

 tive transformation by any process whatsoever, al- 

 though numerous attempts have been made to d« 

 so. These radioactive changes, therefore, seem 

 to offer a reliable means of estimating certain 

 periods of time. Among the radioactive changes 

 appears one in which the metal uranium trans- 

 forms itself into the metal lead and into the gas 

 helium. The rate of transformation is such that 

 five per cent, of a quantity of uranium would 

 change into lead and helium in about three 

 hundred and seventy millions of years. If, there- 

 fore, we determine the amount of uranium, lead 

 and helium in a mineral we can form an idea as 

 to how long these elements have been in contact 

 with each other. Estimates that have been made 

 from the quantities of helium in uranium ores 

 vary between eight and seven hundred million 

 of years, according to locality. Since some of 

 the helium (it being a gas) may have leaked out 

 of the ores, these intervals of time must be re- 

 garded as minimum estimates only. Calculations 

 based on the quantity of lead in uranium ores 

 vary from three hundred and forty to one thous- 

 and seven hundred millions of years according to 

 locality. Here another complication appears. 

 All the different kinds of lead do not come from 

 uranium. Only lead of atomic weight, about two 

 hundred and six, may be regarded as produced 

 from uranium. Until, therefore, it has been de- 

 termined exactly what the atomic weights of the 

 lead in the various ores really are we must con- 

 sider the estimates as maximum estimates only. 

 The atomic weight of the lead in a few ores has 

 been found to be very close to two hundred and 

 six. In one of these the age of the mineral has 

 been estimated at a little over nine hundred mil- 

 lions of years. The calculation of the age of the 

 uranium deposits rests upon the laws of nature as 

 we now believe them to be. It would be a waste 

 of time to speculate on future discoveries or upon 

 a possible evolution of natural law. The calcu- 

 lated ages are the lengths of time during which 

 we may suppose the chemical elements to have 

 been in more or less close mechanical contact with 

 each other. They do not represent the time that 

 has elapsed since the earth may have reached a 

 state capable of supporting organic life as we 

 now know it. 



Arthur W. Goodspeed, 



Secretary 



