280 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 19, 



of the valleys and inequalities, previously existing before they were 

 filled up with loess, were partially or entirely re-excavated, and some 

 new ones formed*. 



Having since the publication of these views had an opportunity of 

 examining the loess of the basin of the Mississippi, — a formation sin- 

 gularly identical in mineral characters and in the genera of its in- 

 cluded shells, — my conclusions respecting the nature of the European 

 loess and its mode of origin have been confirmedf . 



The phsenomena can be best explained, both in Europe and North 

 America, by supposing a gradual subsidence of dryland to the extent 

 of several hundred feet during the deposition of the fluviatile mud, 

 and an equally gradual upheaval of the same tract during the period 

 when extensive denudation was effected. In the same manner as the 

 southern and seaward termination of the loess is somewhat abrupt in 

 Louisiana, where it bounds the delta of the Mississippi, so the cor- 

 responding mass of upraised freshwater mud in Belgium ceases some- 

 what suddenly along a well-marked line, which has been traced out by 

 MM. d'Omalius d'Halloy, Dumont, and others, running east and west 

 nearly along the 51st parallel of latitude, by Cologne, Juliers, Lou- 

 vain, Audenaerde, and Courtray in Belgium, to Cassel, near Dunkirk, 

 in France. "Whether this line indicates the original extent of a 

 fluviatile formation, or whether it was produced by the denudation of 

 a deposit once stretching farther north, I am unable to decide. Its 

 absence beyond the line alluded to is of great importance to a geo- 

 logist who is examining the tertiary strata of the low countries bor- 

 dering the sea, for it allows them to appear at the surface, except 

 where they are concealed by fine sand, much resembling the dunes 

 on the coast. Everywhere to the south of lat. 51° the loess greatly 

 impedes the geological observer, not only by its continuity and 

 thickness, but also by producing a soil so favourable to agriculture, 

 that sand-pits, excavations for clay, or quarries of building-stone, 

 paving-stone, limestone, and other materials, if opened, are almost 

 always filled up immediately, in order that no space may be lost to the 

 farmer. The geologist, therefore, in a country where natural sections 

 are rare, is often reduced to such artificial ones as happen to be 

 exhibited during the workings of a single year. 



In regard to the relative ages of the loess and the northern drift, 

 with its erratics, the only positive information which I obtained 

 during this tour was on crossing the Meuse from Maestricht to the 

 right bank of that river, opposite the city. Here, in company with 

 M. Van Rymsdyck, I observed that the sands of the Limburg tertiary 

 series were covered by a bed of quartzose gravel with erratics, and this 

 again by loess 30 feet thick. The locality alluded to is the table-land 

 of Rassburg, near Geulem, which is about 300 feet above the Meuse, 

 and about 450 feet above the level of the sea. The erratics are some 

 of them very angular, and more than 2 feet in diameter, consisting of 

 quartzose slate, similar to that of the Ardennes, from which they are 

 believed to have been transported. 



* See Proceed. Geol. Soc. vol. ii. p. 83 ; and Edinb. New Phil. Journ. July 1833. 

 t Second Visit to the United States, vol. ii. p. 294 ; and ibid. chap, xxxiv. 



