Formations on the Left Bank of the Rhine. 158 



from the slow erosion of the stream by which it discharged it- 

 self. In the last case, the sides of the basin are cut through 

 by this natural emissary, as is seen in the Meerfekler and the 

 Drieser maare, as well as in those near Strohn and Walsdorf. 

 In the other case, the regularity of the basin has been more 

 or less destroyed by the bursting of its banks, and considera- 

 ble deposits of trass, or rather of peperino, have been formed, 

 evidently aggregated by means of water. Examples of this 

 are met with in the remains of craters near Steffler, Schalken- 

 meyrener, and Rockeskill. On those points where lava has 

 been emitted in a liquid form, a regular crater is rarely to be 

 seen ; at least at the source of the lava-current. There exists, 

 however, always one or more such craters in the vicinity of this 

 source, which appear to have produced violent aeriform ex- 

 plosions, and ejected scoriae and ashes, while the lava was 

 flowing from the neighbouring orifice. The force and rapi- 

 dity of these explosive discharges of confined vapours, is at- 

 tested by the great size of the cavities they have hollowed out 

 of the solid grey wacke strata. Thatof Meerfeld, for instance, one 

 of the largest, measures above 500 feet from the surface of the 

 lake, (which is itself 150 feet in depth,) to the average height 

 of the ridge which encircles it, and its diameter can fall very lit- 

 tle short of a mile. The quantity of fragmentary ejections heap- 

 ed round these basins is not at all proportionate to their ex- 

 tent. The greater part consists of slate and sandstone, in 

 pieces of every size, and appearing half-burnt, probably from 

 having fallen repeatedly upon the surface of lava within the 

 vent whence the explosions of vapour were discharged. 



The accompanying sketch of a map of part of this district (see 

 1'late IV.) will convey an idea of the relative disposition, and pe- 

 culiar characters of these sites of volcanic activity. The most 

 westerly point on which any traces of volcanic eruption are met 

 with, is Ormont, where, upon the wild and elevated transi- 

 tion plateau of alternating slate and quartz-rock, two small 

 cones are seen to rest. They are in contact at their bases, and 

 have neither craters nor visible lava-currents. The scoria; and 

 fragments of which they are composed are basaltic, with much 

 augite and large plates of brown mica. Isolated crystals and 

 pieces of augite also occur, some nearly as large as the fist. 



