EVOLUTIONAL GEOLOGY. 299 



eiu'h otlnn-. Nor are the}' opposed to conclusions drawn from a studv 

 of stratified rocks; thus Sir Archibald Geiki(% 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, 

 l)y a little sti-etching and a little paring, to bring these various estimates 

 into closer harmon3^ 



Professor Darwin's estimate is admittedly a minimum; the actual 

 time, as he himself expressly states, "may have been much longer." 

 Lord Kelvin's estimate, which he would make nearer twenty than forty 

 miliions, 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 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 

 toward 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 that 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 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 suggests the contrary, 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 uniformly distributed 2 mm. thick per century — yet 

 we have ever}'^ reason to believe that volcanoes are but the supei-ficial 

 manifestation of far greater bodies of molten material which lie con- 

 cealed 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 exten- 

 sive 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 l)een wholly by conduction, but also tosouie 

 slight extent by convection, and to a still greater extent by the bodily 

 migration of liquid lava from the deeper layers of the crust toward the 

 surface. 



The existence of local reservoirs of molten rock witliin the crust is 

 still more important in another connection — that is, in relation with the 



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

 conveyed by conduction. 



