668 ANTIQUITY OF FOSSIL BIRDS. 



within the period alluded to, was very unequal. In the course of the 

 interval between E 1 and E 2, strata of micaceous shale and sandstone 

 of the system D, more than 3,000 feet thick, were deposited ; and dur- 

 ing the accumulation of this immense mass of rock some species dis- 

 appeared, while many survived and are common to d 4 and d 5 ; other 

 fossils being peculiar to each of those subdivisions respectively. 



Trap rocks accompany the " Colonial beds" E 1, and are decidedly 

 of contemporaneous origin. Occasionally an orthoceras may be seen 

 involved in the greenstone, while pebbles and angular fragments of trap 

 are intermixed with the fossils of the colony. 



Again, there are other intrusions of similar igneous rocks at the base 

 of E 2, and M. Barrande with good reason appeals to these volcanic 

 appearances as lending support to his hypothesis of former changes of 

 level, by which a barrier of land may have been lowered for a time so 

 as to allow currents of salt-water flowing from the northeast to intro- 

 duce the fauna E 1 into the region previously occupied by D ; and a 

 recurrence, he remarks, of similar oscillations may afterwards have 

 caused the retreat of the colonists, as well as the subsequent return of 

 most of them when the fauna E 2 obtained its permanent footing in 

 Bohemia. Warm currents, like the Gulf Stream, pouring into a colder 

 sea, might carry with them a whole assemblage of species fitted for a 

 more elevated temperature, and capable of superseding the natives of a 

 colder sea, while colder currents invading a warmer sea might give rise 

 to analogous phenomena. In each case along the edges of the space 

 thus colonized, some members of the old native fauna might maintain 

 their ground against the new-comers ; and this may explain why, when 

 the deposit E 1 thins out to a few inches, some species of D are inter- 

 mingled with those of E 1. 



It may be useful to add that in E 2 (a calcareous formation only 500 

 feet in thickness), no less than 900 species of fossil invertebrata have 

 been found by M. Barrande. This set of strata passes upwards into F, 

 and this again into G, and G into H, each having, at the point of con- 

 tact, so many species in common, that M. Barrande has thought it 

 necessary to regard the whole as one system ; yet such is the aggregate 

 result of continual changes, that when the two extremes of the series 

 are contrasted, there is only 1 per cent, common to E 2 and H. 



Many important conclusions will follow if we admit the accuracy of 

 the facts and reasonings above set forth. M. Barrande has himself 

 remarked, that, before his discoveries were made, a geologist, finding in 

 some part of Europe to the northeast of Prague, rocks characterized by 

 the fossils of E 1, would certainly have regarded them as Upper Silu- 

 rian, instead of assigning them to their true era, viz. that of D or the 

 Lower Silurian. On the other hand, if the fauna D, after it was locally 

 exterminated in the region of Prague, still continued to flourish else- 

 where under a slightly modified form which might, in accordance with 

 M. Barrande's nomenclature, be styled d 6 — such a fauna might cer- 

 tainly be mistaken for one of Lower Silurian date, although, in truth, 



