MAARS. 223 



chon has ingeniously observed, " to belong in their formation 

 to about the same epoch as the eruption of the lava streams 

 of the true volcanoes. Both are situated in the vicinity of 

 deeply-cut valleys. The lava-producing volcanoes were de- 

 cidedly active at a time when the valleys had already at- 

 tained very nearly their present form ; and we also see the 

 most ancient lava streams of this district pouring down into 

 the valleys." The Maars are surrounded by fragments of 

 Devonian slates, and by heaps of gray sand and tufa mar- 

 gins. The Laacher lake, whether it be regarded as a largo 

 Maar, or, with my old friend C. von Oeynhausen, as part of 

 a large caldron-like valley in the clay-slate (like the basin 

 of Wehr), exhibits some volcanic eruptions of scoria? upon 

 the ridge surrounding it, as is the case on the Krufter Ofen, 

 the Veitskopf, and Laacher Kopf. It is not, however, mere- 

 ly the entire want of lava streams, such as are to be ob- 

 served on the Canary Islands upon the outer margin of true 

 craters of elevation and in their immediate vicinity — it is 

 not the inconsiderable elevation of the ridge surrounding the 

 Maar, that distinguishes this from craters of elevation ; the 

 margins of the Maars are destitute of a regular stratifica- 

 tion of the rock, falling, in consequence of the upheaval, con- 

 stantly outward. The Maars sunk in the Devonian slate 

 appear, as has already been observed, like the craters of 

 mines, into which, after the violent explosion of hot gases 

 and vapors, the looser ejected masses (Rapilli) have for the 

 most part fallen back. As examples I shall only mention 

 here the Immerather, the Pnlvermaar, and the Meerfelder 

 Maar. In the centre of the first mentioned, the dry bottom 

 of which, at a depth of two hundred feet, is cultivated, are 

 situated the two villages of Ober- and Unter-Immerath. 

 Here, in the volcanic tufa of the vicinity, exactly as on the 

 Laacher lake, mixtures of feldspar and augite occur in sphe- 

 roids, in which particles of black and green glass arc scat- 

 tered. Similar spheroids of mica, hornblende, and augite, 

 full of vitrified portions, are also contained in the tufa veins 

 of the Pulvermaar near Gillenfeld, which, however, is en- 

 tirely converted into a deep lake. The regularly circular 

 Meerfelder Maar, covered partly with water and partly with 

 peat, is characterized geognostically by the proximity of the 

 three craters of the great Mosenberg, the most southern of 

 which has furnished a stream of lava. The Maar, however, 

 is situated 639 feet below the long ridge of the volcano, and 

 at its northern extremity, not in the axis of the series of 



