298 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 22, 



Brown-coal, and have been convulsed, contorted, and broken up by 

 penetrating dykes and outbursts of basaltic and other igneous rocks. 

 Again the alternate action of subsidence was introduced, and the 

 northern portion of this Brown-coal became submerged under the 

 waters of the northern sea, inasmuch as we find the Brown-coal 

 again overlaid by the marine deposits of Westeregelu and Magdeburg. 

 But it is almost premature to enter on this question ; I have not 

 myself yet visited the locality. 



We have thus faintly attempted to trace the possible history of the 

 origin of the Mayence basin formations, and of their gradual rise until 

 the waters in which they were deposited had reached their highest 

 level. It only remains, in conclusion, to say a few words respecting 

 their probable drainage and the desiccation of the Rhine Valley. 

 Assuming that no outlet existed for its waters, the surface-level of the 

 lake must at last have reached a height at which they would over- 

 flow the mountain-barrier by which they were enclosed. This spot 

 appears to have been at the commencement of the narrow gorge near 

 Bingen, by which the Rhine now escapes from the Rheingau, and 

 flows through the most picturesque and striking scenery of Northern 

 Europe, between Bingen and Coblentz. The formation of this gorge 

 was in the first instance probably owing to volcanic agency. The 

 mountain-chains of the Hundsriick and the Taunus were already 

 partially riven asunder, and the long pent-up waters, finding at 

 length an outlet, rushed with overwhelming force through the 

 chasm, removing all opposing obstacles, and carrying with them a 

 large portion of those deposits which had previously formed the 

 bottom of the lake. The imagination is bewildered in contemplating 

 the details of this stupendous operation ; but it is not unreasonable to 

 suppose that something similar, if not indeed of a grander character, 

 took place during the process of cutting back this mighty channel, 

 to what we now see in the St. Lawrence, while cutting back its 

 channel from Lake Ontario to Lake Erie. It is vain to attempt to 

 calculate the time which must have elapsed during this operation, 

 when we consider the hard nature of the Devonian schists through 

 which the waters had to cut their way ; but when we add to this, 

 that a similar lacustrine basin exists between Coblentz and Neuwied, 

 below which again the river has cut its way through another mighty 

 gorge, from Andernach to the Siebengebirge, we can only wonder at 

 the countless ages which must have gone by during this compara- 

 tively small period of the tertiary epoch, itself one of the shortest of 

 the many successive ages which the study of the crust of our globe 

 has brouarht to our knowledge. 



