Mr. HoRNEU on the Geology of the Environs of Bonn. 473 



the hope of obtaining millstones^ like those of Nieder Mendig; but it decom- 

 poses readily when exposed to the air_, and masses of sufficient size without 

 rents could not be had. 



Fragments of grauwacke and of quartz are common in the lava ; they are 

 fast imbedded in it, and their surfaces are frequently vitrified. These vitrified 

 pebbles are met with abundantly among the scoriae on the sides of the hill, 

 varying in size from a foot to the tenth of an inch in diameter. " Any one/' 

 says M. Thomae, " who had not picked them up himself on the spot from 

 " among the brown cinders, might swear they came out of a porcelain furnace." 

 p. 17. Volcanic bombs are sometimes found, and some which M. Thomae 

 " broke presented the following remarkable appearance. " Three almost per- 

 " fectly round balls, the largest of which might be about the size of a man's 

 '' head, contained in the interior rapilli, small rounded portions of porous lava, 

 " vitrified and non-vitrified quartz pebbles, and small fragments of grauwacke, 

 " some friable, others vitrified, and some unchanged. The bodies, which were 

 " of the size of nuts, filled the whole interior space of the bomb ; they lay chiefly 

 " loose, often five or six were adhering together, or single ones were fixed to 

 " the sides of the bomb, so that in breaking the porous lava shell, which was 

 " about a finger's breadth in thickness, great caution was necessary to prevent 

 " the contents falling out. In the interior of one of the bombs there was a 

 " small detached crystal of augite." p. 21. 



V. Pages 450 and 465. 



In Leonhard and Bronn's Neues Jahrbuch fur Miner alogie, ^c, for 1835, 

 p. 678, there is the following communication from Professor Noggerath to 

 the Editors. 



"Bonn, 30 October, 1835. 



"Hitherto no shells have been found in our Rhenish brown-coal. Recently 

 " some freshwater shells were brought to me from a bed which seems to be in- 

 '' termediate between earthy brown-coal and bituminous (brown-coal) clay. 

 " This bed, which is of inconsiderable thickness, covers the alternations of 

 " compact clay ironstone, which occur near Rott, eastward of the Siebenge- 

 '' birge, an account of which is given in my Rheinland IVestphalen, iv. p. 388. 

 " You will see by the specimens sent herewith that the shells are changed into 

 " a kind of brown-coal, and are almost quite flat. KM. Bronn will venture to 

 " pronounce an opinion upon these, I should wish you to insert it along with 

 '*this notice in your journal." 



M. Bronn adds, '' All these shells belong to the genus Planorbis, but they 

 " are so crushed as to be almost wholly unrecognisable. The largest and best- 



