34 GEOLOGICAL MEMOIRS. 



Second Fauna possess. Although in the Upper Silurian division 

 the number of the genera is barely half as great as in the Second 

 Period, yet the numerical maximum of the species is found at the 

 base of this group, as already noticed by Murchison. This great 

 development is strongly contrasted with the very limited geographical 

 surface, compared with the wide extent of the Second Fauna, which 

 is occupied, especially in Europe, by the Third Fauna. 



"We shall find, however, a like contrast between the vertical extent 

 of these faunas, the Second Fauna comprising groups of deposits of 

 very great magnitude in proportion to those of the Third. This is 

 shown in my ideal section of the Silurian rocks of Bohemia in the 

 Bulletin, Soc. Geol. France, Jan. 1851. In England the whole Upper 

 Silurian Division is very limited ; in Sweden it is represented by 

 Gothland ; in Russia, by the Oesel Island and a few other patches ; in 

 France, by the district of St. Sauveur le Vicomte and of Feuguerolles, 

 which are but little islands in the Silurian area. On the whole how- 

 ever, the Third Fauna, although vertically and horizontally confined 

 in narrow limits, is characterized in Europe by its richness, not alone 

 in Trilobites, but also in species of all classes of MoUusca. 



In America, on the other hand, the Third Fauna occupies (accord- 

 ing to De Verneuil) at least quite as much extent as the Second. 



The succession of these two faunas is clearly shown in England by 

 the succession of their respective deposits, and still more clearly in 

 Bohemia. The boundary-lines which divide them in the British 

 Isles have been precisely indicated by Murchison, who has also 

 pointed out the same in Sweden, Russia, &c. ; and De Verneuil has 

 recognized them in America. But in none of these countries is the 

 exact limit so sharply drawn as in Bohemia, where an outburst of 

 trap, spreading over the surface of the Silurian basin, has at once cut 

 off the whole Second Fauna ; this Period, therefore, being separated 

 from the Third Fauna by an azoic plutonic horizon. With the excep- 

 tion, therefore, of the outlying "colonies*," which have conditions 

 peculiar to themselves, there is scarcely to be found in the whole of 

 this district a single fossil common to the two successive faunas. This 

 condition having resulted from local causes, as just mentioned, it is 

 not likely to be so clearly indicated in other places, and in England 

 especially we have a considerable number of known species passing 

 over from the one to the other division. What exact conditions 

 obtain in other districts, I do not know, the requisite information on 

 the subject being still wanting ; but in all cases we may regard the 

 Second and Third Faunas as constantly separate and independent. 



It only remains therefore for us to determine the upper limit of the 

 Third Fauna. I am not, however, fully prepared to point out any 

 absolute line of demarcation, from want of sufficiently extensive ob- 

 servations, especially in districts where the observer can at once survey 

 a considerable succession of true Silurian beds overlaid by well-cha- 

 racterized Devonian rocks. America will without doubt some day 

 furnish the data required to solve this question, which all European 

 districts still leave unsettled. 



This is certain, that in Sweden, Bohemia, and France, at the base 

 * Vide infra, p. 37. 



