388 



the valley of the Rhine, which sent off large branches up the courses 

 of the Mayne, Neckar, and other tributary valleys, in all of which 

 large patches of loess are occasionally met with. The barrier of 

 such a lake has been placed in the narrow gorge of the Rhine between 

 Bingen and Bonn ; but this theory is untenable, as there are proofs 

 of the loess having once filled that gorge, and of its having over- 

 spread the adjoining hills of the Lower Eifel; also that it reached 

 to the flanks of the hills bounding the valley of the Rhine as far 

 down as Cologne and still further. 



Instead of supposing one continuous lake of sufficient extent and 

 depth to allow of the simultaneous accumulation of loess at all heights 

 and throughout the whole area where it now occurs, I conceive that 

 subsequently to the period when the countries now drained by the 

 Rhine and its tributaries, acquired nearly their actual form and geo- 

 graphical features, they were again depressed gradually by a move- 

 ment like that now in progress on the west coast of Greenland. In 

 proportion as the whole district was lowered, the general fall of the 

 waters between the Alps and the ocean was lessened, and both the 

 main and lateral valleys, becoming more subject to river inundations, 

 were partially filled up with fluviatile silt containing land and fresh- 

 water shells. After this operation, when a thickness of many hun- 

 dred feet of loess had been thrown down slowly, and in the course of 

 many centuries, the whole region was once more upheaved gradu- 

 ally, but perhaps not equally, throughout the whole region. During 

 this upward movement most of the fine loam was carried off by 

 denudation to such an extent that the original valleys were nearly 

 re-excavated. The country was thus restored to its pristine state, 

 with the exceptionof those patches of loess still remaining, and which, 

 from their frequency and their remarkable homogeneousness of 

 composition and fossils, attest the original continuity and common 

 origin of the whole. By introducing such general fluctuations of re- 

 lative level, we may dispense with the necessity of erecting and after- 

 wards removing a great barrier more than 1200 feet high, sufficient 

 to exclude the ocean from the valley of the Rhine during the accu- 

 mulation of the loess. 



Dr. Fitton has again brought before us those curious phsenomena 

 in the Island of Portland from which the former alternate existence 

 of sea, of dry land, and lastly, of a body of fresh water in the same 

 place, all anterior to the formation of the chalk, has been clearly in- 

 ferred. In the ancient soil, called in Portland, the " Dirt bed," the 

 silicified trunks of trees and their roots are still preserved. Some 

 curious facts are just published on this subject in the new Part of our 

 Transactions, in a memoir by Dr. Buckland and Mr. De la Beche. 

 After Mr. Webster had first made known the nature and existence 

 of the dirt bed. Professor Henslow ascertained that between this 

 and the marine oolite of Portland there were two other beds of car- 

 bonaceous clay, and in one of these Dr. Fitton has now found the 

 remains of Cycadese, from which it appears that the forest of the 

 dirt bed was not the first vegetation which grew on this tract. First 

 there must have been the sea of the oolite, then land which sup- 



