1919.] Benson.—Some Estimates of the Age op the Earth. 67 
as the doctrine of uniformity supposed, but have been fluctuating. Such 
fluctuation, he thinks, has been rhythmical, and has been growing more 
rapid and more extreme. When we study the sedimentary record of such 
a continent as North America we see how preponderating in the past were 
the conditions of wide-spreading shallow seas and lands of relatively smaller 
area and lower relief than at present, which would make smaller contribu¬ 
tion of detrital sediment or dissolved salt to the ocean. The present time, 
therefore, should be one of abnormally large land area and high relief, for 
the earth has but recently passed through a stage of continental emergence 
and the building of immense folded ranges. Sedimentation and weathering- 
are now unusually rapid. Moreover, he contends, the earlier geological 
estimates fail to take proper account of the great dependence of sedimenta¬ 
tion on continenta 1 areas upon steady subsidence, and the same material 
was probably deposited and removed many times before it came finally to 
rest. There is thus not sufficient allowance for the innumerable gaps, 
small or great, that appear in the record. The occurrence of fairly 
rhythmical fluctuations of conditions of climate and land-elevation during 
the past is inferred from the fluctuating changes of the sediments that were 
laid down. Hence he concludes that the sedimentary record, and the 
saline contents of the ocean, so far from opposing the physico-chemical 
age-estimates, are, if rightly interpreted, rather in support of such 
immense figures. The discussion is a most interesting one, but perhaps 
the reader may be pardoned if he feels that at times the argument is 
rather forced. 
If we grant, then, that the earth has an age of over 1,000 millions of 
years, we are brought, as Professor Barrell shows, face to face with a still 
greater problem—the age of the sun. If the sun’s heat is maintained, as 
was formerly supposed, by the gravitational condensation of its parts, it 
could not have continued its present rate of heat-radiation for more than 
50 millions of years. Yet the geological record shows that from the earliest 
times of which we have adequate information the fluctuations of tempera¬ 
ture on the earth’s surface have been within the present narrow range of 
climatic conditions, though the glacial or hot and arid conditions have been 
sometimes more widespread, sometimes less so, than at present. The 
superficial atmospheric temperatures on the earth appear to depend almost 
entirely on the solar radiation. Either, then, the estimate of the age of 
the earth given is many times too great, or the hypothesis of gravitational 
contraction as the source of solar heat does not account for more than a 
very small fraction of the solar radiation. “ Even if we consider the sun’s 
heat largely the result of radio-active transformations, by no possibility can 
more than one-third of it be accounted for by the changes of radium and 
thorium. We must postulate other and unknown atomic and molecular 
disintegrations supplying more heat. Such suppositions only emphasize 
our ignorance of the cosmic processes. The scheme of the universe is more 
profound, and the unknown a little nearer than it was recently thought 
to be.” 
Here, then, is the great difficulty. If the age-estimate thus deduced 
from radio-active transformations is correct we are thrown back upon diffi¬ 
culties greater than were seen before. One recalls the encouraging words 
