455 



the production of trachyte, which is so intimately connected with 

 these ancient lake-craters, he has direct recourse to the analogies of 

 modern volcanos, and also attempts the reconstruction of those more 

 recent currents of basaltic lava, of which there are now left such im- 

 perfect evidences. If Dr. Hibbert has succeeded in proving the relative 

 age of the outbursts of the various volcanic products of the Eifel from 

 trachyte to the most modern basaltic ejections, he has accomplished a 

 task from which his precursors have shrunk ; their great difficulty 

 consisting in the comparative absence or obscurity of all strata of se- 

 condary or tertiary age, which, if they contained distinct evidences in 

 their organic remains, might have been deemed true historical re- 

 cords. In Auvergne and in the Cantal, where no such deficiencies 

 exist, but where, on the contrary, the sedimentary strata have been 

 elevated into mountain-masses teeming with the remains of organic 

 life ; the precise relative periods at which the intensity of volcanic 

 action has been renewed, or suspended, is demonstrable by alternate 

 dislocations and regularities of the associated strata. But in the 

 Eifei, if we except the fossils of that very ancient group of rocks the 

 grauwacke, the evidences to be gathered from organic remains in the 

 subsequent epochs are deplorably deficient, being merely observable 

 in thin patches of brown coal and tertiary clay, a few only of which 

 are connected with the volcanic phenomena of this district. 



That brown coal is associated with tertiary deposits of various 

 ages is well known to those who have explored Germany and the 

 flanks of the Alps ; and the greater part of this mineral in the basin 

 of the Lower Rhine has been referred to an early period in the ter- 

 tiary series. This subject has recently been freed from much of its 

 obscurity by the observations of our valued fellow-labourer Mr. Leo- 

 nard Horner, on the Geology of the Environs of Bonn*. From this 

 very able Memoir we learn, that notwithstanding the difficulty of 

 assigning a precise geological age to this deposit, on account 

 of the almost entire absence of shelly remains, yet from the im- 

 bedded fishes, frogs, and plants, which though essentially differing 

 from, bear a strong analogy to existing species, the brown coal of 

 the Rhine is probably of the age of the lacustrine limestone of 

 Aix en Provence. 



Mr. Horner further throws new light upon the period of the tra- 

 chytic and basaltic eruptions of the Sieben-gebirge, which, like many 

 volcanic hills in central France, he supposes to have burst forth 

 from beneath an ancient lake ; and whilst he indicates that this ridge 

 has been elevated posterior to the formation of the associated brown 

 coal, he shows that one of the lake-craters on the opposite bank of 

 the Rhine, the Rodderberg, was formed during a more recent period, 

 probably contemporaneous with the accumulation of the loess or loamy 

 alluvium. 



We are here naturally led to reflect upon that exciting theoretical 

 question concerning craters of elevation, which now divides thegeo- 



* M. Mitscherlich is also, I am rejoiced to learn, now engaged in writing 

 a Memoir upon this district. 



c 



