162 



KNOWLEDGE 



[Septembeu 1, 1892. 



period since the earth's consolidation to 10 or 15 millions 

 of years.'" 



We pass on to the argument from the tides. It is 

 generall.v admitted that the daily tidal waves must, in some 

 degree, diminish the rate of rotation of the earth on its 

 axis. Its action has been compared to that of a brake on 

 a wheel. At one time, then, the rotation was more rapid ; 

 in other words, the earth's day was shorter, and has since 

 been steadily getting longer. If we assume any antiquity 

 for the globe greater than 100 million years, he thinks 

 the flattening at the poles would be greater, owing to 

 greater centrifugal force having been formerly exerted by 

 the more rapid rotation. 



Lastly, Lord Kelvin has attempted calculations based 

 upon the radiation of heat from the sun, and also upon the 

 amount of heat generated by the falling together of meteoric 

 masses, such as by clashing together may have given . rise 

 to the sun. He admits, however, that his conclusions 

 from this source are, from the nature of the case, less 

 reliable. Still, like the other calculations, they point to a 

 comparatively small number of millions of years, perhaps 

 about twenty. The sun may, however, have continued to 

 receive showers of meteorites, and thus to be replenished 

 with heat ; which would disturb these calculations. More- 

 over, certai-n chemical changes may be the means of 

 liberating heat in the sun. 



But we will not dwell on these difficulties here. It is 

 hardly necessary to say that most geologists consider that 

 conclusions such as these are too sweeping. Seeing what 

 vast changes have taken place on the earth — so many 

 thousands of feet of solid rock formed by slow deposition 

 in water, so many new forms of life introduced at certain 

 epochs, while others were extinguished — the geologist 

 cannot bring himself to believe that all the changes (only 

 fully realized by those who study the record of the rocks) 

 could have taken place within 20 or even 100 millions of 

 years. Some, doubtless, would demand much more time, 

 and refuse to accept even the limit of 100 milhons. No 

 one distrusts the actual calculations ; but many do 

 seriously distrust the data (or want of data) upon which 

 they are founded. Hence a serious difference has arisen 

 between geologists and physicists with regard to the 

 duration of geological time. Mathematics are an excellent 

 mill, and will grind out beautiful results ; but what you 

 get out of this mill depends very much on what you put 

 into it, and if you put in material based upon imcertain 

 assumptions, you must not be surprised at getting a result 

 tainted with similar uncertainty. 



Let us quit this somewhat unsatisfactory region of 

 speculation, and see what further light can be gained from 

 the science of geology. It will be interesting to compare 

 any results that may be obtained with those above 

 mentioned, and to see whether they harmonize. 



The geologist knows only two time-keeping processes ; 

 one is rock fonnntion (deposition), the other ruck destruction 

 (denudation). A third is sometimes referred to, namely, 

 changes in the organic world, involving the appearance, 

 from time to time, of new species, genera, families and 



* There is some uncertainty as to tlie amount of heat radiated by 

 the sun. Assuming Helmboltz's theory as to the source of the sun's 

 heat, that is that it is due to the slow contraction of tlie sim's bulk, and 

 assuming that the sun has derived its heat solely in this ivay, we may say 

 with some certainty that the contraction of the sim to its present size 

 from a diameter as large as the earth's orbit would not ha\e furnished 

 more than 20,000,000 times as much heat as the sun now supplies in a 

 year. But from theoretical considerations it seems probable that a 

 gaseous mass losing heat by radiation, and contracting under its own 

 gravity, must rise in tempei-ature. So that itispossiblethat theannual 

 loss of heat in former times may have been less than at present, even 

 thou"h the radiating surface •iras then greater. — A. C. Eanyaed. 



orders of plants and animals — changes which are compre- 

 hended under the one word " evolution." But this kind of 

 change, which has been going on ever since the oldest 

 (Archfean) rocks were first formed, concerns the biologist 

 more than the geologist. The biologist, as Professor 

 Huxley said, has no clock, and must take his time from 

 the geological clock. In other words, when, on passing 

 from one rock formation to another, a great change in the 

 fossils is noticed — as, for instance, in passing from 

 Primary rocks to Secondary or from Secondary to Tertiary, 

 the lapse of time required to bring about such evolutionary 

 change can only be gauged by the thicknesses of the 

 strata in which the different fossils are found, and partly, 

 in the two cases above quoted, by the " stratigraphical 

 break " between the two sets of strata; that is, the amount 

 of rock denuded during the interval between the two eras. 

 As the Greeks used to detect " the lazy foot of time " by 

 the slow dropping of water from a clepsydra, so the 

 geologist measures his periods by the work of water, either 

 as a rock destroj'er or as a rock former. This is our 

 water-clock, and our two measures of time are (1) depth of 

 rock denuded, (2) depth of rock deposited. Now the 

 condition of the water-clock's accuracy as a time-keeper 

 was uniformity of action, that the drops should continue 

 falling at the same rate ; so with the geological clock. 

 These two processes, so closely related to each other, must be 

 supposed to have been working throughout geological time 

 (that is, the time during which the great series of stratified 

 rocks were being formed) with considerable unitormity. 

 This brings us back to the "theory of uniformity" 

 originated by the illustrious Hutton, and expanded and 

 explained by Playfair and Lyell. 



Eeaders of Knowledge will hardly need to be told that 

 " dBuudation '' is chiefly efi'ected by " rain and rivers.'' 

 The consequence of denudation in one place is rock 

 formation in another ; the one is complementary to the 

 other. In other words, the clcbrix of continents is carried 

 by rivers into lakes, seas and estuaries, there to settle 

 down and " sow the dust of continents to be." Now rivers 

 depend for their supplies on rainfall; hence, rainfall is one 

 of the main factors in problems about denudation. 

 Geologists believe (from a mass of evidence in the 

 stratified rocks which it would take too long to expound 

 here) that the rainfall has, in past periods, been pretty 

 much what it is nou- in various parts of the world — 

 not necessarily in Europe. It inaij, however, have been 

 somewhat greater as far back as the Archajan and 

 PaliEOZoic times, when, perhaps, the earth was sensibly 

 warmer and the sun sensibly hotter. Hence, geologists 

 consider that they are justified in attempting to form some 

 kind of estimate of former periods of time from the two 

 processes above referred to. Not only is it possible thus 

 to compare one period with another and to say which was 

 the longest, but we venture to think that it is justifiable to 

 attempt to calculate the limits of geological time on the 

 basis of the rate at which strata may be formed. We 

 want to translate feet of rock formed into years. To give 

 a mathematical basis to geology is one of the great 

 problems of the future. What degree of success awaits 

 such efforts we cannot say, but certain attempts have been 

 made to gauge denudation, and to see at what rate it goes 

 on. With regard to deposition of strata, very little, if 

 anything, has been done, and we cannot help thinking that 

 important results might be obtained in this direction ; but 

 of that we shall speak presently. 



Let us briefly consider the first operation, namely , the wear- 

 ing away of land. The subject of atmospheric denudation 

 has been arithmetically investigated, in order to ascertain 

 at what rate a given contiaent, or portion of a continent, 



