METHODS OF PALEOGEOGRAPHY 447 



Chattanooga, etcetera, the known faunas of which are of the nekton and 

 plankton type. As a rule, limestones are indicative of seas of wider extent 

 among low lands during times of moist and warmer temperature, while 

 dolomites mark about the same conditions, but in shallower, evaporating 

 seas. Oolites are formed in the littoral region of seas between tides where 

 the lime salts accrete about a nucleus due to its repeated wetting and 

 drying, and otherwise. 



The interpretation thus given the various kinds of sediments has been 

 applied in the construction of the present maps, but not with the same 

 care as that given the faunas and the areal geology. Volcanoes and vol- 

 canic material have also been considered, but information regarding these 

 has been plotted in only a few of the more striking times and areas of 

 eruption. The positions of these are shown by asterisks. 



DIASTROPHIC METHOD 



Having ascertained the essential periods of emergence and transgression 

 by the faunal method, the diastrophic principle was then used to fix the 

 major time divisions. Taken by itself, the latter method is believed to 

 be nearly as unreliable, where permanent results are concerned, as the 

 petrologic method. In fact, the principle of diastrophism can rarely be 

 used before taking the fossil evidence into account, for it is the latter that 

 fixes and determines physical events. Diastrophism, however, is of much 

 value in paleogeography, but it must follow, not precede, the evidence 

 furnished by the fossils. 



Continental Seas, or Negative Continental Elements 

 (See map, plate 48) 



All the Paleozoic seas now engaging the attention of American stratig- 

 raphers are of the "continental" type — that is, their deposits have been 

 furnished by shallow seas within "great continental basins/ 7 This funda- 

 mental generalization was first announced by Dana in 1856, 34 was repeated 

 in 1863, 35 and was clearly defined in 1874. 36 The later term — "epicon- 

 tinental seas" — of Chamberlin and Salisbury 37 has the identical meaning 

 of continental seas. 



The "Interior Continental region" is subdivided by Dana as follows: 

 "(1) The Eastern interior east of the Cincinnati uplift; (2) the Central 



34 Dana : American Journal of Science, vol. 22, 1856, pp. 335-349. 



35 Dana : Manual of Geology, 1863. 



30 Dana : Ibidem, second ed., 1874, pp. 145-146. Also Bull. Geological Society of 

 America, vol. 1, 1890, p. 41. Manual of Geology, fourth ed., 1895, p. 461. 

 87 Chamberlin and Salisbury : Geology, vol. 1, 1904, p. 11. 



