June 29, 1900.] 



SCIENCE. 



1007 



shrinking at a definite rate, and a compu- 

 tation based on this rate has told how many 

 millions of years ago the sun's diameter 

 should have been equal to the present di- 

 ameter of the earth's orbit. Manifestly 

 the earth can not have been ready for habi- 

 tation before the passage of that epoch, 

 and so the computation yields a superior 

 limit to the extent of geologic" time. 



Before passing to the next division of the 

 subject — the computations based on rhythms 

 — a few words may be given to the results 

 which have been obtained from the study 

 of continuous processes. Realizing that 

 your patience may have been strained by 

 the kaleidoscopic character of the rapid re- 

 view which has seemed unavoidable, I shall 

 spare you the recitation of numei-ical de- 

 tails and merely state in general terms that 

 the geologists, or those who have reasoned 

 from the rocks and fossils, have deduced 

 values for the earth's age very much larger 

 than have been obtained by the physicists, 

 or those who have reasoned from earth 

 cooling, sun cooling and tidal friction. In 

 order to express their results in millions of 

 years the geologists must employ from 3 to 

 5 digits, while the physicists need but 1 or 

 2. When these enormous discrepancies 

 were first realized it was seen that serious 

 errors must exist in some of the observa- 

 tional data or else in some of the theories 

 employed ; and geologists undertook with 

 zeal the revision of their computations, 

 making as earnest an effort for reconcilia- 

 tion as has been made a generation earlier 

 to adjust the elements of the Hebrew cos- 

 mogony to the facts of geology. But after 

 rediscussing the measurements and read- 

 justing the assumptions so as to reduce the 

 time estimates in every reasonable way — 

 and perhaps in some that were not so reason- 

 able — they were still unable to compress 

 the chapters of geologic history between the 

 narrow covers of physical limitation ; and 

 there the matter rests for the present. 



The rocks which were formed as sedi- 

 ments show many traces of rhythm. Some 

 are composed of layers, thin as paper, 

 which alternate in color, so that when 

 broken across they exhibit delicate band- 

 ing. In the time of their making there 

 was a periodic change in the character of 

 the mud that settled from the water. 

 Others are banded on a larger scale ; and 

 there are also bandings of texture wherie 

 the color is uniform. Many formations 

 are divided into separate strata, as though 

 the process of accretion had been periodic- 

 ally interrupted. Series of hard strata are 

 often separated by films or thin layers of 

 softer material. Strata of two kinds are 

 sometimes seen to alternate through many 

 repetitions. Borings in the delta of the 

 Mississippi show soils and remains of trees 

 at many levels, alternating with river silts. 

 The rock series in which coal occurs are 

 monotonous repetitions of shale and sand- 

 stone. Belgian geologists have been so 

 impressed by the recurrence of short se- 

 quences of strata that they have based an 

 elaborate system of rock notation upon it. 



Passing to still greater units, the large 

 aggregates of strata sometimes called sys- 

 tems show in many cases a regular sequence, 

 which Newberry called a ' circle of depo- 

 sition.' When complete, it comprises a 

 sandstone or conglomerate, at base, then 

 shale, limestone, shale and sandstone. This 

 sequence is explained as the result of the 

 gradual encroachment, or transgression, as 

 it is called, of the sea over the land and its 

 subsequent recession. 



In certain bogs of Scandinavia deep ac- 

 cumulations of peat are traversed horizon- 

 tally by layers including tree stumps in 

 such way as to indicate that the ground 

 has been alternately covered by forest and 

 boggy moss. The broad glaciers of the Ice 

 age grew alternately smaller and larger — 

 or else were repeatedly dissipated and re- 

 formed — and their final waning was char- 



