14 Missing Chapters of Geological History. 



groups are never actually repeated after an interval represented 

 by rocks containing another group — that on the whole, as we 

 recede from any geological horizon we depart more and more 

 widely from the collection of organic forms that seem to cha- 

 racterize it, it becomes clear that breaks in the succession of 

 strata are important and positive facts indicating lapses of time 

 and well deserving careful study. 



It is comparatively easy to trace such breaks in any one 

 country after the first working out of the detailed geology of 

 a country. It is, however, far more difficult to estimate their 

 relative and positive value and construct a chronology from 

 this sort of evidence. Nor is it easy to establish sound and 

 useful comparisons between the succession and breaks in dif- 

 ferent countries which have been exposed to very different 

 influences during the lapse of geological time. Any approach 

 to so desirable a result is to be welcomed, and the elaborate 

 tables recently published with respect to the Secondary rocks 

 are of permanent value in this respect. 



The old and familiar tripartite division of rocks into Palaso- 

 zoic, Secondary or Mesozoic, and Tertiary or Cainozoic, is the 

 best illustration of the more important and easily recognized 

 of the breaks in succession. It was long a received doctrine 

 that total and abrupt change in all forms of life marked the 

 transition from any one to any other of these. More recently 

 it has been admitted that there are a few species that pass from 

 the older to the newer, but the number of such species is small 

 in proportion. These great geological landmarks are still em- 

 ployed, and are as available now as they were when first pro- 

 posed. The earliest interpretations of this method of grouping 

 admitted and even assumed the change in species to be due to 

 change of condition ; but the change was regarded as essen- 

 tially and of necessity cataclysmic, or belonging to some 

 paroxysmal disturbance, destroying all that existed before and 

 succeeded by a new creation. 



Geologists have not altogether given up this view. Some 

 of our French contemporaries have even within a few years 

 insisted that every species in each formation was limited to one 

 time and place, and the great maxim that " fossils are charac- 

 teristic of formations^ has thus been carried to an extreme. 

 English geologists have been inclined to admit rather the pro- 

 bability of gradual change, and where there is marked differ- 

 ence in the kind of animal and vegetable life they believe that 

 some intermediate bed has been deposited and removed, or 

 that a long interval elapsed during which there were no deposits. 

 It is these breaks that we have now to consider, so far as England 

 is concerned. 



For the most part marine deposits certainly represent con» 



