294 A. J. Juhes-Broicne — On Rocli-Clamfication. 



we find no break, but a continuous series divisible into three groups 

 ■with distinct faunas, and each entitled to rank as a system. 



As in this instance, so in others, the breaks in the succession of 

 one district are certain to be filled up elsewhere by transitional beds, 

 and as these may contain a fauna of high systematic value, we must 

 always be prepared for the possible intercalation of another system 

 between any two groups that are divided by a decided unconformity. 



Systems must therefore be founded on the evidence of fossils in 

 some area where a continuous succession of marine deposits is foimd; 

 a system may perhaps be defined as a series of strata deposited 

 during the continuance of a certain set of generic forms in any given 

 part of the world. Such a definition will probably be found suffi- 

 ciently elastic, and yet definite enough to secure an approximate 

 equality of value for the several systems. The proportion of special 

 characteristic genera must be variable, but probably it is about one- 

 third of the total number of genera in the faunas of the Palaeozoic 

 and Mesozoic systems. 



It must be conceded at once that systems founded on the differ- 

 ences in successive faunas will not include anything like equal 

 thicknesses of rock. If, as appears most probable, the differentia- 

 tion of species and the development of higher forms has progressed 

 in a constantly increasing ratio, it is clear that the time-value of 

 systems based on such systems will become less and less as we 

 approach modern times. It is certainly a fact that the same forms 

 of life extend through a much greater thickness of rock in the earlier 

 than in later geological times, and they also appear to have had 

 a wider geographical distribution. There is no reason for supposing 

 that the production of rocks went on more rapidly (at any rate during 

 later Palseozoic periods) than at the present time ; limestones must 

 always have been of slow and gradual growth, and yet the persist- 

 ence of species through the Carboniferous Limestone Series is truly 

 remarkable, while the limestones of the Jurassic and Cretaceous 

 systems (though much less thick) are divisible into numerous zones, 

 each characterized by species which do not occur in the next. The 

 changes in the forms of life inhabiting any marine area being thus 

 more rapid in later geological times, an equal amount of change 

 would naturally be accomplished in less time, and consequently if 

 systems are to be measured by paleeontological differences, the newer 

 systems must include less thickness of rock. 



But even when allowance is made for this, it is quite impossible 

 to regard the divisions of Tertiary time introduced by Sir Charles 

 Lyell as being of the same relative importance as -the systems of 

 older date with which they are usually ranked. This feeling has 

 been exj^ressed by Dr. Blanford, who notices the proposal of certain 

 Swiss and German geologists to make two Tertiary systems, — an 

 Eocene (comprising the Oligocene) and a Neogene (comprising the 

 later groups). Dr. Blanford, however, thinks that there is " so little 

 probability of this classification being adopted, that the only practic- 

 able suggestion appears to be to unite the Oligocene with the 

 Miocene, and to include the Pleistocene, which has no claim to recog- 



