November 16, 1900.] 



SCIENCE. 



753 



immeasurable would be the advance of our 

 science could we but bring the chief events 

 which it records into some relation with a 

 standard of time ! 



Before proceeding to the discussion of 

 estimates of time drawn from a study of 

 stratified rocks let us first consider those 

 which have been already suggested b}' other 

 data. These are as follows : (1) Time 

 which has elapsed since the 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, max- 

 imum estimate by Professor J. Joly. 



It may be at once observed that these 

 estimates, although independent, are all of 

 the same order of magnitude, and so far 

 confirmatory of each other. ]S"or are they 

 opposed to conclusions drawn from a study 

 of stratified rocks ; thus Sir Archibald 

 Geikie, in his address to this Section last 

 year, affirmed that, so far as these were 

 concerned, one hundred 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 ; the actual time, as he him- 

 self expressly states, ' may have been much 

 longer.' Lord Kelvin's estimate, which he 

 would make nearer twenty than forty mil- 

 lions, is founded on the assumption that 

 since the period of the ' consistentior status ' 

 the earth has cooled simply as a solid body, 

 the transference of heat from within out- 

 wards having been accomplished solely by 

 conduction.* 

 . It may be at once admitted that there is 



* The heat thus brought to the surface would 

 amount to one-seventeenth of that conveyed by con- 

 duction. 



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 in- 

 fluence of the tides it behaves as though it 

 possessed a rigidity approaching that of 

 steel, and under sudden blows, such as 

 those which give rise to earthquakes, with 

 twice this rigidity, as Professor Milne in- 

 forms me. Astronomical considerations 

 lead to the conclusion that its effective 

 rigidity has not varied greatly for a long 

 period of past time. 



Still, while fully recognizing these facts, 

 the geologist knows — we all know — that the 

 crust of the earth is not altogether solid. 

 The existence of volcanoes by itself sug- 

 gests the contrary, and although the total 

 amount of fluid material which is brought up 

 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 

 uniformly distributed 2 mm. thick per cen- 

 tury ; yet we have every reason to believe 

 that volcanoes are but the superficial man- 

 ifestation 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-con- 

 tinued denudation, are merely the upper 

 portion of more extensive masses which lie 

 remote from view. The existence of molten 

 material within the earth's crust naturally 

 awakens a suspicion that the process of 

 cooling has not been wholly by conduction, 

 but also to some slight extent bj' convection 

 and to a still greater extent by the bodily 

 migration of liquid lava from the deeper 

 layers of the crust towards the surface. 



The existence of local reservoirs of molten 

 rock within the crust is even still more im- 

 portant in another connection, that is, in re- 

 lation with the supposed ' average rate of 

 increase of temperature with descent below 



