406 HISTORICAL GEOLOGY. 



already received local names extend into the adjoining, the introduction of 

 new names in the latter is a wrong to the science. 



5. In all cases, the characteristics of the species and the beds should be 

 carefully scrutinized, lest abruptness due to local migrations (as those caused 

 by slight changes of depth or currents and kinds of sea-bottom) should be 

 mistaken for abruptness of real importance. 



Physical and Organic Breaks. — Prominent among the events influencing 

 the rock-structure and life of a continent is that of mountain-making. The 

 Appalachian Mountains stand as a grand time-boundary between the Paleozoic 

 seon and the Mesozoic ; and cotemporaneous orographic movements make a 

 like limit in European geology. Moreover, it was attended by the most 

 remarkalale of organic breaks. The Taconic mountains mark the close of 

 the Lower Silurian, an epoch of abrupt change in North America; and 

 parallel disturbances occurred in Britain and Europe. The Laramide or 

 post-Cretaceous mountain system along the Eocky Mountains is another 

 such boundary for America, separating Mesozoic and Cenozoic time, though 

 V not_as_complete in the attendant organic break as in the physical. But it so 

 happens that no corresponding event occurred at this time in Europe, the 

 orographic movements most nearly synchronous taking place after the com- 

 mencement of Cenozoic time. Nevertheless, the organic break at the close 

 of the Cretaceous period is even greater for Europe than for America. 

 Such a fact seems to show that there was some other catastrophic event 

 concerned ; but its nature is yet to be studied out. 



Part of the breaks referred to above were limited in their effects to 

 the hemisphere including America, Europe, northern and middle Asia, and 

 northern Africa. The opposite hemisphere, that of India, Australia, and 

 South Africa, has been more or less independent, althovigh the two were alike 

 in many characteristics ; and owing to this, the boundary closing Paleozoic 

 time, so strongly marked in the geological history of Europe and America, 

 cannot be satisfactorily defined in the latter. The coal period is of later 

 date than that of Europe and America, it occurring in the Permian, and the 

 Permian period blends with the Triassic. 



Such orographic time-boundaries are registered not only in the rocks that 

 are upturned, but also in unconformabilities between them and the succeed- 

 ing rocks. It is important to note, however, as already stated, that the 

 unconjormahility exists only in upttirned regions. A short distance away, the 

 succeeding beds will be found lying conformably over the same kinds that 

 are upturned in the mountains ; moreover, the organic break there may be 

 less pronounced, and in more distant regions it may fail altogether. The 

 unconformability is, however, none the less important as a time-boundary, 

 for orographic upturnings have been events of great geographical extent 

 after long ages of preparation. 



The Subdivisions. — The several grades of subdivisions of geological 

 time are named (1) J5ons, (2) Eras, (3) Periods, (4) Epochs ; and the 

 corresponding terms applied to the formations are Series, Systems, Groups, 



