276 KEY TO TERRESTRIAL HISTORY 



fore suspend our judgment while awaiting further 

 discoveries.* 



The immediate motive which prompted geologists in 

 their efforts to carry the identification of strata down to 

 such extreme limits was their desire to determine the 

 structure of the earth's crust, and to read its history in 

 greater detail ; and so far as their efforts have met with 

 success, so far does the fundamental assumption upon 

 which they have proceeded meet with its justification. 



As a direct consequence of a stricter study of the dis- 

 tribution of fossils, the whole aspect of this discussion 

 has undergone a change : the general correspondence 

 of sedimentary systems to periods of time can no longer 

 be in doubt ; our difficulties would seem to be restricted to 

 the consideration of zones, t This will almost appear self- 

 evident if we picture to ourselves the case as it would 

 arise if we accept for a moment the startling assertion of 

 Huxley, and admit the simultaneous coexistence of three 

 sedimentary systems. Let us select for one of these the 

 Jurassic system, then it is not a single fauna common 

 to the whole, but the thirty-three distinct faunas of its 

 several zones, with which we have to deal. If now we 

 add the next system below, the Trias, this means so 

 many more faunas ; and again more when we take into 

 account the next system above, the Cretaceous. Having 

 then chequered the earth with these multifarious faunas, 

 we have next to conceive of some method by which they 

 may be made to succeed each other in all parts of the 

 world in an unvarying order, and this seems to my mind 



* In a work which has appeared as these sheets are passing through 

 the press, Mr. Rudolf Ruedemann has shown that the succession in 

 Australia is in strict conformity to that which prevails elsewhere 

 (" Graptolites of New York," N.Y. State Museum, Memoir 7). 



f For an interesting discussion of this subject, see J. E. Marr, 

 l< The Principles of Stratigraphical Geology," 1898, p. 50, and Proc. 

 Camb. Phil. Soc., 1898, vol. vi. p. 74. 



