512 NATURAL SCIENCE. s^pt.. 



so that there were neither great oceans nor isolated continents in our 

 modern sense of the terms. From this condition of geographical 

 equality, it seems to me there has been a gradual tendency to the 

 development of high continental plateaux, while the deeper sea- 

 troughs have been merged into oceans, and the volume of the surface 

 waters has been continually increased by the extrusion of the water- 

 substance from the interior of the earth. 



On this hypothesis we can at once explain the rarity of extensive 

 oceanic deposits of the modern type among the great series of 

 stratified rocks, for in the comparatively narrow and shallow seas of 

 Palaeozoic time no such deposits could be formed. Now, as a matter 

 of fact, while deep-water muds are common among Palaeozoic rocks, 

 no rock which is likely to have been analogous to a modern abyssal 

 ooze has yet been found, for the radiolarian chert of Ayrshire can 

 hardly be claimed as such, either in character, thickness, or extent. 

 Coming down to Mesozoic time, we find the Cretaceous Chalk, which 

 does in all three respects bear great resemblance to a modern 

 calcareous ooze, but it is not associated with abyssal deposits of the 

 red clay and radiolarian ooze types. When, however, we reach 

 Tertiary times the West Indian deposits furnish proof that the 

 oceans were deep enough for the formation of all the modern types of 

 oceanic deposits. 



On this hypothesis of the gradual evolution of oceans and 

 continents we can imagine the former existence of continental land 

 where the ocean now rolls, without depressing any modern continent 

 below the ocean to produce it. In short, we may believe that the 

 places of our continents have nearly always been occupied by land or by 

 shallow seas without assuming that the land areas have always been 

 restricted to the present continental plateaux. We may likewise 

 believe in the great antiquity of the deeper parts of the oceans, 

 without assuming that these oceans have always been either as deep 

 or as extensive as they now are. 



In his Appendix (p. g), Mr. Fisher thinks it probable that his con- 

 clusions as to the structure of the sub-oceanic crust do not apply to the 

 borders of the oceans, and that the one type of crust changes gra- 

 dually into the other type, " so that there is an intermediate belt of 

 which we cannot assert that it belongs exclusively either to the 

 oceanic or to the continental type." This allows us a certain amount 

 of latitude ; but I do not think he goes far enough, and believe his own 

 conclusions must eventually lead him into the very theory of evolution 

 which I have just suggested. For instance, he accounts for the 

 greater density of the upper layer of the typical sub-oceanic crust by 

 supposing it to consist of extravasated basic lavas ; but the extrava- 

 sation of these lavas must have gone on pari passu with the com- 

 pression of the continents, and hence the very construction of a special 

 type of sub-oceanic crust must have been a process of gradual develop- 

 ment. In dealing with geological time, Evolution, as distinct from 



