THE POSITION OF GEOLOGY. 545 



the earth is not less than from a thousand to twenty-five hundred 

 miles thick is open to question. We can not imagine that a crust 

 of that enormous thickness could, in such recent geological times, 

 have possessed so great a flexibility as is indicated by the move- 

 ments we have referred to. Independently of that improbability, 

 there are certain geological facts which are inexplicable on that 

 assumption. Volcanic phenomena would be unintelligible ; for 

 vents traversing that thickness of solid rock could hardly be kept 

 open owing to the cooling which the lava in its ascent would 

 undergo. The rock fragments ejected during explosions are also 

 those of rocks which lie at no great depth, while, with the in- 

 crease of temperature in descending beneath the surface, there is 

 every reason to suppose that at a depth to be measured by tens, 

 and not by hundreds of miles, the immediate underlying magma 

 at least is in a state of plasticity such as would allow of compara- 

 tively free movements of the crust. Again, surely, if the crust 

 were so thick, we might expect to find, when that crust was broken 

 and its edges thrust up by compression or protrusion of the igneous 

 rocks, that some indications of that enormous thickness should be 

 exhibited ; but none such are forthcoming. Whatever may be the 

 state of the nucleus, there is nothing geologically to indicate, 

 as some physicists also have contended on other grounds, that 

 the outer crust of the earth is more than from about twenty to 

 thirty miles thick. The effective rigidity will therefore, if it 

 be necessary, have to be explained in some other manner than 

 that of a comparatively solid globe or of a crust of enormous 

 thickness. 



We are thus brought face to face with apparently irrecon- 

 cilable opinions. That they admit of adjustment there can be no 

 doubt, but it must be by mutual understanding. How it is to be 

 effected is a problem for the future. 



These, briefly, are the barriers which restrict inquiry on many 

 important questions. On the side of the uniformitarians, it is 

 assumed that every position must be reduced to a fixed measure 

 where fixity is not possible of time and speed ; and, on that of 

 the physicists, geologists are gently reminded that the subject is 

 outside their immediate sphere of inquiry, in a way somewhat 

 suggestive of " the closure." 



It would be an unfortunate day for any science to have free 

 discussion and inquiry barred by assumed postulates, and not by 

 the ordinary rules of evidence as established by the facts, how- 

 ever divergent the conclusions to which those facts lead may be 

 from the prevailing belief. In any case it must be remembered 

 that no hypothesis can be true which does not satisfy the con- 

 ditions both of the geological phenomena and of the physical 

 laws. 



VOL. XLIV. 42 



