CARBONIFEROUS SYSTEM. 345 



Section 3. — Carboniferous System. — Age of Acrogens and 



Amphibians. 



Retrospect. — Before taking up in detail this important and interest- 

 ing age, it will be instructive to glance back over the ground traversed, 

 and draw some conclusions. 



If we compare, in physical geography, the American with the Eu- 

 ropean Continent, we find the one marked by simplicity and the other 

 by complexity of structure. This is true not only of the map-outline, 

 but also of the profile-outline, or orographic structure. Now, as history 

 furnishes the key to social and political structure, so geology furnishes 

 the key to physical structure. The American Continent — at least in its 

 eastern part — has developed comparatively steadily from the Laurentian 

 nucleus southward and eastward, and probably northward. We have 

 already seen how the Silurian area was added to the Laurentian, and 

 the Devonian to the Silurian. It shall be our pleasure, hereafter, to 

 show the continuance of this steady development throughout the whole 

 geological history. For our knowledge on this interesting subject we 

 are indebted almost wholly to Prof. Dana. 



In the case of America, the continent thus sketched in outline in 

 the earliest times has been steadily worked out in detail throughout all 

 subsequent time ; with some very considerable oscillations, it is true, de- 

 termining unconformability of strata, rapid changes of physical geog- 

 raphy and climate, and therefore of species, thus marking the great 

 divisions of time, but on the whole without change of plan or wavering 

 of purpose ; in the case of Europe, on the contrary, geological history 

 consists of a series of oscillations so great that it amounts to a successive 

 making and unmaking of the continent. 



Hence, nearly all geological problems are expressed in simpler terms, 

 and are more easily solved, here than there. Hence, also, while in Eu- 

 rope the ages and periods are separated by unconformability of the 

 rock-system, as well as change in the life-system, in America they are 

 separated mainly by change in the life-system only. 



Subdivisions of the Carboniferous System and Age.— The Carbonifer- 

 ous age is subdivided into three periods, viz. : 1. Sub-Carboniferous ; 

 2. Coal-measures, or Carboniferous proper ; 3. Permian. 



The sub-Carboniferous was the period of preparation ; the Coal- 

 measures the period of culmination ; the Permian the period of decline 

 and transition to the Mesozoic. The whole thickness of the carbon- 

 iferous strata in Nova Scotia is 16,000 feet ; in South Wales it is 14,000 

 feet; in Pennsylvania 9,000 feet, and in Lancashire 16,336 feet.* 



The sub-Carboniferous consists mainly of marine formations ; the 



* "Hawkins, Nature, vol. xxxviii, p. 449, 18S8. 



