ANNIVEESAEY ADDEESS OP THE PRESIDENT. XCV 



which he collected in those islands, and which I have referred to 

 in my notice of the work of Dr. Hartung. I will only observe, in 

 conclusion, that there is not a chapter in the book which does not 

 contain much new and valuable matter. 



Herr von Dechen, so well known for the active part he has 

 taken in the preparation of the G-eological Map of Grermany, has 

 published at Bonn a very important work on the Lake of Laach 

 and its volcanic neighbourhood. The chaotic manner in which 

 volcanic rocks of different ages and of various mineralogical cha- 

 racters are heaped together in this district, renders it a most 

 difficult task to disentangle their history, and we are therefore all 

 the more indebted to Herr von Dechen for the effort he has made 

 to solve the many difficult problems here met with. I will there- 

 fore endeavour to point out some of the more important geological 

 conclusions which the author has arrived at. 



The products of the volcanos in the neighbourhood of the Lake 

 of Laach are in close contact with the lower division of the Devo- 

 nian beds which form the far-extending basis of all the other for- 

 mations in this district ; also with the middle Tertiary (Oligocene) 

 deposits, or the Brown-coal formation, which, as a whole, does not 

 extend beyond the limits of this volcanic district, although with 

 interruptions it spreads out far beyond it ; and also with the high- 

 level gravels which extend in terraces to the valley and the bed of 

 the Rhine, and with the overlying loam and loess. 



The time of volcanic action began before the close of the Oli- 

 gocene period, and posterior to the period when the Devonian beds 

 had by elevation acquired their present highly inclined position, 

 and its surface had already undergone its most important modifi- 

 cations. 



The great hollowing out in the district of the Devonian beds in 

 the neighbourhood of this volcanic region of Coblenz andBendorf, 

 as far down as Anderuach and Fahr, was already in existence 

 before the formation of the Brown-coal deposits. 



The formation of the Ehine Valley took place after the deposit 

 of the Brown-coal beds. 



Some of the volcanic outbursts, e.g., that which furnished 

 the material for the tufa, with impressions of leaves, near Plaidt, 

 are older than the formation of the valley. Other outbursts, on 

 the other hand, belong to the most recent changes which this 

 district has undergone. The series of volcanic outbursts here 

 comprises a long period of time, during which the formation of 

 the valleys and the gradual development of the form of the surface 

 took place. 



On the confines of the volcanic district some basaltic hills 

 appear, the emergence of which, as in the Siebengebirge, probably 

 took place during the formation of the Brown-coal. 



The streams of lava which have flowed down the neighbouring 

 valleys prove the previous existence of these valleys, and also that 

 the form of the sux'facc of the whole neighbourhood has undergone 

 no important changes from that time down to the present day. 



