Mr, Horner on the Geology of the Environs of Bonn. 455 



contains yellow, brown, semi-opal, not to be distinguished from that which I 

 have described as occurring in the quartzose conglomerate at Tanzchen, and 

 like it passing into the mammillary structure of chalcedony. It contains 

 numerous casts of Planorbes, Lymnei, and Paludinas, and jointed stalks of a 

 plant. I have examined above a hundred fragments, to try to discover Gyro- 

 gonites, but have found none, Mr. Lyell informs me that M. Deshayes, to 

 whom he showed some of these specimens, found in them the three freshwater 

 shells above named : he said that they do not look like modern tertiary shells, 

 that the Planorbis appears, as far as can be determined by a cast, identical 

 with P. rotundatus ; that the Lymneus seems to be longiscatus, both occur- 

 ring in the Paris basin. The plant is described by Professor Lindley to be 

 fragments of dicotyledonous wood ; and, in one of the specimens, there is a 

 fragment of some organic body, which may have been the fruit of the palm, 

 of which the nucleus is left entire, and the fleshy integuments half broken 

 away. 



This Marienforst siliceous rock has not been noticed by any previous 

 writer on this district, probably because it has not been long known, as 

 nothing of the kind exists in any other part of the district. If it had been at 

 one time an extensive deposit, subsequently broken up, one could hardly fail 

 to find fragments of it in the vast mass of gravel which covers the plateau; 

 but this is not the case. On the other hand, I know of no rock in the ad- 

 joining country from which these blocks could have been derived ; and had 

 they come from a distance, it is not probable that they would have been 

 confined to so small a spot. Although I do not mean to adduce it as a proof 

 of identity, I would point out the almost identical semi-opal found in this 

 rock and in the quartz conglomerate at T'anzchen. Upon the whole I am 

 inclined to consider this as a small local deposit belonging to the same period 

 as the brown -coal formation, and very possibly produced by some hot spring 

 in the volcanic region holding silica in solution. 



The brown-coal formation is covered by a mass of gravel almost through- 

 out its whole extent, there being very few places where it is wanting. It is 

 of very variable thickness, sometimes being only a thin covering, at others 

 as much as eighty feet deep ; and M. Von Dechen, in the memoir already 

 quoted, desci-ibes a mass of sand which he considers as belonging to the 

 gravel in the neighbourhood of Ichendorf, near Bergheim, of 123 feet in 

 thickness. The gravel consists of rounded fragments of grauwacke and 

 quartz, the latter greatly predominating, the pebbles generally small, about 

 the size of a walnut ; but very often large blocks, both of grauwacke, quartz, 

 and basalt are imbedded in it. The pebbles are united by an argillaceous 



