CONTINUITY OF THE CHALK. 



493 



not only a continuity of chalk-formation, but also to some extent of the 

 chalk-fauna, to the present time. 



These facts were certainly unexpected, but, so far from shaking the 

 foundations of geological science, as some have imagined, they are in 

 perfect accordance with the fundamental principles of geological suc- 

 cession properly understood ; as we now proceed to show : 



1. The facts of identity have been exaggerated. Many of the 

 Foraminifera only are identical. Among Echinoderms the identity 

 is generic 4 not specific. 2. In comparing higher with lower species, 

 we find that the lower species are widely distributed both in space 

 (geographically) and in time (geologically), and that the continuance 

 or range in time becomes less and less in proportion as we rise in the 

 scale. Fig. 828 is constructed to illustrate this point ; we see that liv- 



Fig. 828— Diagram illustrating the Relative Duration of Lower and Higher Species. 



ing species of mammals extend back only a little way into the Quater- 

 nary, living species of mollusks back to the beginning of the Tertiary, 

 while living species of Foraminifera, as we might expect, extend back 

 into the Cretaceous. 3. There is a necessary relation between fauna 

 and external conditions. Changes in the latter determine correspond- 

 ing changes in the former. Now, deep-sea conditions are evidently 

 far less subject to change — far more continuous — than shallow-water 

 and land conditions. For this reason, we should expect deep-sea faunas 

 to change very slowly. 4. But this can not affect the geological chro- 

 nology, because this chronology rests almost wholly on the remains of 

 shallow-water and land animals. Chalk is the only profound sea- 

 bottom formation certainly known. It is, therefore, wholly exceptional. 

 5. The reason it is exceptional is that, as a broad general fact, the pres- 

 ent continents have been, through all geological times, steadily heaved 

 upward out of the ocean, growing larger and higher ; and, therefore, 

 the successive additions have been nearly always shalloio marginal 

 bottoms and shallow interior seas. That the exception should occur 

 in Europe rather than in America, too, is in keeping with the general 

 character of the development of the European as contrasted with the 

 American Continent. Chalk is also found in Texas ; but here also was 

 a deep interior sea, an extension of the Mexican Gulf. 6. Conversely, 

 the fact that chalk is so exceptional is proof of the development of 



