STRATIGRAPHICAL GEOLOGY. 507 



independent fauna, which owes its origin to a special act of 

 creation, and is clearly distinguishable from the preceding and 

 succeeding faunas. It happens in rare cases that species are 

 continued into a higher complex of strata than that in which 

 they took origin, but these cases occur only when the higher 

 strata succeed conformably upon the lower, in other words, 

 when no marked crust-disturbance has taken place between 

 the two periods of deposition. Thus D'Orbigny thought it 

 possible to base stratigraphy wholly upon palaeontological 

 features, more especially upon the occurrence of Mollusca, 

 Echinodermata, and Ccelenterata. 



He commenced a great palgeontological work, which was 

 intended to supply a description of all the fossil forms in 

 France belonging to these three divisions of the animal 

 kingdom. The gigantic scope of the work was too much 

 even for such an enthusiastic worker as D'Orbigny. Between 

 1840-55 several volumes of D'Orbigny's Paleontologie Fran^aise 

 appeared, comprising descriptions of the Jurassic and Cretaceous 

 Cephalopods, part of the Gastropods from the same two 

 systems, and the Cretaceous Brachiopods and Hippurites, 

 Irregular Echinids, and Bryozoa. In two other works, the 

 Elementary Course of Palaeontology (1849-52) and the Pro- 

 drome of Stratigraphical Palaeontology (1850-52), D'Orbigny 

 elucidated his sub-division of stratified rocks and his views on 

 Stratigraphical geology. 



He divided fossiliferous rocks into six periods or Terrains, 

 and sub-divided the first five periods into twenty-seven groups 

 (etages). He selected the names of characteristic localities for 

 the designation of the groups of strata, and followed Thurmann's 

 example in adding the affix "ien" to give uniformity to the 

 series. D'Orbigny was thoroughly familiar with the Mesozoic 

 faunas but knew less about those of other epochs, and he 

 made the mistake of assigning to the Mesozoic faunas a much 

 greater significance in his Stratigraphical succession than to 

 the Palaeozoic or Cainozoic faunas. He discarded the terms 

 Palaeozoic, Mesozoic, and Cainozoic, and assumed an equal 

 value for the twenty-seven successive groups which he 

 distinguished. In accordance with his sub-division of the 

 rock-succession, D'Orbigny supposed that organic creation 

 had been completely renewed twenty-seven or twenty-eight 

 times. 



The chief merit of D'Orbigny's works is their remarkable 



