718 REPOKT— 1900. 



Separation of the earth and moon, fifty-six millions of years, minimum estimate by 

 Professor G. H. Darwin. (2) Since the ' consistentior status,' twenty to forty 

 millions (Lord Kelvin). (3) Since the condensation of the oceans, eighty to 

 ninety millions, maximum estimate by Professor J. Joly. 



It may be at once observed that these estimates, althougli independent, are all 

 of the same order of magnitude, and so far confirmatory of each other. Nor are 

 they opposed to conclusions drawn from a study of stratified rocks; thus Sir 

 Archibald Geilde, in his Address to this Section last year, alfirmed tliat, so far as 

 these -were concerned, 100 millions of years might suffice for their formation. 

 There is then very little to quarrel about, and our task is reduced to an attempt, 

 by a little stretching and a little paring, to bring these various estimates into 

 closer harmony. 



Professor Darwin's estimate is admittedly a minimum ; tlie actual time, as he 

 himself expressly states, ' may have been much longer.' Lord Kelvin's estimate, 

 which he would make nearer twenty than forty millions, is founded on the 

 assumption that since the period of the ' consistentior status ' the earth lias cooled 

 simply as a solid body, the transference of heat from within outwards having been 

 accomplished solely by conduction. 



It may be at once admitted that there is a large amount of truth in this 

 assumption ; there can be no possible doubt that the earth reacts towards forces 

 applied for a short time as a solid body. Under the influence of the tides it behaves 

 as though it possessed a rigidity approaching tliat of steel, and under sudden blows, 

 such as those which give rise to earthquakes, with twice this rigidity, as Professor 

 Milne informs me. Astronomical considerations lead to the conclusion that its 

 etfective rigidity has not varied greatly for a long period of past time. 



Still, wliile fully recognising these facts, the geologist knows — we all know — that 

 the crust of the earth is not altogether solid. The existence of volcanoes by 

 itself suggests the contrarj% and although the total amount of fluid material which 

 is brought from the interior to the exterior of the earth by volcanic action may be, 

 and certainly is, small — from data given by Professor Penck, I estimate it as 

 equivalent to a layer of rock imiformly distributed 2 mm. thick per century ' — 

 vet we have every reason to believe that volcanoes are but the superficial manifesta- 

 tion of far greater bodies of molten material which lie concealed beneath the 

 ground. Even the wide areas of plutonic rock, which are sometimes exposed to 

 view over a country that has suffered long-continued denudation, are merely the 

 upper portion of more extensive masses which lie remote from view. The existence 

 of molten material within the earths crust naturally awakens a suspicion that 

 the process of cooling has not been wholly by conduction, but also to some slight 

 extent by convection, and to a still greater extent by the bodily migration of 

 liquid lava from the deeper layers of tlie crust towards the surface. 



The existence of local reservoirs of molten rock within the crust is even 

 still more important in another coucection, that is, in relation with the supposed 

 ' averacre rate of increase of temperature with descent below the ground.' It is 

 doubtful whether we have yet discovered a rate that in any useful sense can be 

 spoken of as ' average. The widely divergent views of ditterent authorities as to 

 the presumed value of this rate may well lead to reflection. The late Professor 

 Prestwich thought a rise of 1°F. for every 45 feet of descent below the zone 

 of constant temperature best represented the average ; Lord Kelvin in his earlier 

 estimates has adopted a value of 1° F. for every 51 feet ; the Committee of 

 this Association appointed to investigate this question arrived at a rate of 1° F. 

 for every 60 feet of descent ; Mr. Clarence King has made calculations in which a 

 rate of i° F. for 72 feet is adopted ; a re-investigation of recorded measurements 

 would, I believe, lead to a rate of 1° F. in 80 or 90 feet as more closely approaching 

 the mean. This would raise Lord Kelvin's estimate to nearly fifty millions of years. 



When from these various averages we turn to the observations on which 

 they are based, we encounter a surprising divergence of extremes from the 



' The heat thus brought to the surface would amount to one seventeenth of that 

 conveyed bj^ conduction. 



