(_g._) _Trass of the Brühl Valley._--The Brühl Valley, which unites with that of the Rhine at the town of that name, and drains the northern side of the volcanic region, has always been regarded with much interest by travellers for the presence of a deposit of "trass" with which it is partially filled. The origin of this valley was pre-volcanic, as it is hewn out of the slaty rocks of the district. But at a later period it became filled with volcanic mud (tuffstein), out of which the stream has made for itself a fresh channel. The source of this mud is considered by Hibbert[12] to have been the old volcano of the Lummerfeld, which, after becoming dormant, was filled with water, and thus became a lake. At a subsequent period, however, a fresh eruption took place near the edge of the lake, resulting in the remarkable ruptured crater known as the Kunksköpfe, which rises about four miles to the north of the Laacher See. The eruptions of this volcano appear to have displaced the mud of the Lummerfeld, causing it to flow down into the deep gorge of the Brühl, which it completely filled, as stated above. On walking down the valley one may sometimes see the junction of the tuff with the slate-rock which enfolds it. The tuff consists of white felspathic mud, with fragments of slate and lava, reaching a depth in some places of 150 feet. After it has been quarried it is ground in mills, and used for cement stone under the name of _trass_. It is said to resemble the volcanic mud by which Herculaneum was overwhelmed during the first eruption of Vesuvius, and which was produced by the torrents of rain mixing with the ashes as they were blown out of the volcano. Sufficient has probably now been written regarding the dormant, or recently extinct, volcanic districts of Europe to give the reader a clear idea regarding their nature and physical structure. Other districts might be added, such as those of Central Germany, Hungary, Transylvania, and Styria; but to do so would be to exceed the proposed limits of this work; and we may therefore pass on to the consideration of the volcanic region of Syria and Palestine, which adjoins the Mediterranean district we have considered in a former page. [1] Daubeny, _loc. cit._, p. 71. The geology of this region has had many investigators, of whom the chief are Steininger, _Erloschenen Vulkane in der Eifel_ (1820); Hibbert, _Extinct Volcanoes of the Basin of Neuwied_, 1832; Nöggerath, _Das Gebirge im Rheinland_, etc., 4 vols.; Horner, "On the Geology of Bonn," _Transactions of the Geological Society, London_, vol. iv. [2] The views of Dr. Hibbert are not inconsistent with those of the late Sir A. Ramsay, on "The Physical History of the Valley of the Rhine," _Quart. Jour. Geol. Soc._, vol. xxx. (1874). [3] Von Dechen, _Geog. Beschreib. des Siebengebirges am Rhein_ (Bonn, 1852). [4] Hibbert, _loc. cit._, p. 18. [5] Horner, "Geology of Environs of Bonn," _Transactions of the Geological Society_, vol. iv., new series.