Sharat K. Roy

“ Since the rate of transformation is known, data for calculating the age of the mineral and with it the rock formation of which it is a part, can be obtained by measuring the quantity of helium and lead in the rock and comparing it with the quantity of uranium in the same volume of material. But, as helium is a gas, it is likely that a certain portion of it leaks out and consequently the estimate of age on the basis of how long helium had been in contact with uranium and lead is to [Pg 9] be regarded as a minimum estimate. ”

“ Any estimate based on the temperature of the earth, or of the sun, encounters similar practical difficulties, for the temperature of a body may not be constant. It may rise or it may fall. Further, the rate of change of temperature is controlled by a variety of conditions, such as the amount of energy radiated, the supply of energy and so forth. Nor is there any record of the immense quantity of heat produced by igneous agencies and radio-activity.
Another estimate, one of the earliest, was based on the rate of life transformation in successive periods. ”

“ The study of the succession of faunas—the change of one species to another, can only indicate the magnitude of time involved. It cannot afford any basis, whatsoever, for a concrete expression of geologic time. ”

“ The radio-active minerals are commonly found in igneous rocks. They are widely distributed all over the world. The parents of the whole series of radio-active elements are uranium and thorium. They possess the highest atomic weights of all known elements. Each of these parental elements transforms itself through a succession of changes. The final product of uranium is the formation of the metal lead and the gas helium. ”

“ It is quite significant that each geological era, occasionally a geological period, has its characteristic grouping of life developed from the life of preceding periods. As we climb higher in the geological column, life becomes more and more complex and specialized. ”

“ Since traces of the lowest forms of life have been found in practically [Pg 2] the oldest known sedimentary strata, the problem of determining the age of life necessarily involves the determination of the age of those strata. But unravelling the dead past is not an easy task. ”

“ In other words, the lead ratios are in keeping with the geological age.
The procedure of applying the lead ratio in calculating geological time can be briefly stated thus: The rate of production of lead from uranium can be readily calculated. The rate at which helium is generated is accurately known and the quantity of lead liberated in the same time is approximately 6.5 times that of helium. In a year one gram of uranium produces 1.25 × 10−10 grams of lead, and at this rate 8,000 [2] million years will be required for the production of one gram of lead. ”

“ The configuration of the earth, its climate, humidity, temperature and many other factors have varied from time to time and with them the rate of erosion, deposition and solution has either accelerated, diminished or ceased. ”

“ A precise answer to such a question is impossible and an adequate one demands a longer time than can usually be afforded. The consequences of inadequate explanation have often proved to be unsatisfactory. ”

“ During the last three decades, a number of radio-active changes of one chemical element into another have been discovered and studies of certain minerals and rocks containing various radio-active elements have created means to calculate their age with remarkable accuracy. ”

“ But estimates based on the lead ratios of radio-active minerals offer results consistent among themselves. That is, whenever fresh, primary, uranium-bearing minerals of the same geological age have been examined, the lead ratios are always found to remain constant. The value of the ratios increases or decreases as the geological age of the respective mineral increases or decreases. ”

“ Detailed descriptions of how the metal uranium slowly and regularly breaks down in a descending series into the metal lead and the gas helium, will be found in the literature on radio-activity. For our purpose, it suffices to say that according to Barrell, an atom of uranium which breaks up will ultimately give rise as a stable product to eight atoms of helium and one atom of lead. ”

“ Furthermore, the rate of deposition along the coast, near the mouth of a large and active river, is much higher than on a coastline where no rivers empty their sediments into the sea. The rate is also largely controlled by the character of the sediments deposited. Sandstone and shale are more rapidly deposited than limestone. ”

“ III figs. 7-9) , have withstood all possible environmental changes and have steadfastly held their own ever since we have records of their very early appearance on earth. On the contrary, we have records of types that have yielded so rapidly to change that their evolution is almost explosive. It is almost incomprehensible how, within such a limited period of time, fishes have changed into amphibians, amphibians into reptiles, and reptiles into birds and mammals (Pl. IV figs. 1-4) . ”