2(12 THE CANADIAN NATURALIST. [Vol. vi. 



tion. which are stated by Barrande to consist chiefly of gray- 

 wackes and grits (" qiiartzites "). Lastly, there are indications 

 that e 1 is superimposed nnconformably upon d 6, in which case 

 the intervitl between the second and third faunas may have been 

 an enormously long one, and some intermediate deposits may be 

 missing.) 



The above definitive introduction, constituting the first phase 

 of the third or Upper Silurian fauna, must have taken place 

 during the deposition of the band e 1, the basement band of the 

 superior division, which agrees lithologically wdth the colonies in 

 being composed of graptolitic schists with calcareous concretions, 

 alternating with sheets of trap. 



It is clear that the interpretation of the facts rests chiefly on 

 the hypothesis of migrations. Most geologists now admit the 

 doctrine of migrations, and Bohemia more than any country pre- 

 sents us with proofs of its truth. 



Thus, M. Barrande has shown that the Bohemian basin of 

 Silurian times was separated by natural barriers from the con- 

 temporaneous ocean which covered the great northern zone of 

 Europe and America. This is shewn by the specific differences 

 between many of the forms (such as the Ceplialo2Joda] of these 

 areas ; but the occurrence of some species common to Bohemia 

 and Northern Europe has also shown that there must have existed 

 temporary communications between these diff'erent regions. Fur- 

 ther, M. Barrande has shown [Mem. sur hi Reapparition dit 

 genre Aretliusinii, 1868,1 that although the colonies are the most 

 striking examples of the intermittent appearance of species in 

 Bohemia, there exists besides in the same b i«iii a considerable 

 number of species equally intermittent, and belonging to difi"erent 

 classes of fossils. This was particularly shown by the occurrence 

 of four Trilobites and one Cephalopod, which existed in c/ 1, at the 

 commencement of the second fauna, completely disappeared during 

 d 2, d 3, and d 4, a.nd reappeared in d 5 at the close of the second 

 fauna, their re ippearance coinciding precisely with the introduc- 

 tion of the colonies into the basin. 



Both these circumstances can be explained by the same hypo- 

 thesis, namely by supposing a temporary communication to be 

 formed between the Bohemian basin and other seas. This hypo- 

 thesis would not only explain the reappearance of the above-men- 

 tioned species after the lapse of a vast period of time, but would 

 also allow of the almost inevitable introduction of various other 

 new forms into the same basin at the same time. 



