232 REACTION OF THE INTERIOR OF THE EARTH 



situated the two villages of Upper and Lower Immerath. 

 Here are found in the volcanic tufa around, just as at 

 the Laacher See, mixtures of felspar and augite form- 

 ing spheres or nodules, in which black and green 

 vitreous particles are interspersed. Similar balls of 

 mica, hornblende and augite, full of vitrifactions are 

 also found in the tufa margin of the Pulver Maar, 

 (near Gillenfeld), which, however, is entirely transformed 

 into a deep lake. The regularly circular Meerfelder 

 Maar, covered partly with water and partly with bog, is 

 geologically distinguished by the vicinity of the three 

 craters of the great Mosenberg, from the southernmost 

 of which a lava-stream has flowed. The Maar is, how- 

 ever, 640 feet lower than the long ridge of the volcano, 

 and is at its northern end ; moreover it is not in the 

 axis of the line of craters, but is more to the north-west. 

 The mean height of the Maars of the Eifel above the 

 sea falls between 922 feet, the height of the Laacher 

 See (if it be regarded as a Maar), and 1588 feet, the 

 height of the Mosbrucher Maar. 



As this is more particularly the place at which to call 

 attention to the general uniformity and accordance in 

 the substances produced by volcanic activity, while 

 there is the greatest diversity in the forms of the out- 

 ward framework belonging to that activity, (as the Maars 

 above spoken of, elevation craters, and conical 

 volcanoes open at the summit,) I will here refer to the 

 striking abundance of crystallised minerals which the 

 Maars brought to the surface at their first explosion, and 

 which now lie partly buried in the tufa. This abundance 

 is particularly great round the Laacher See ; but there 

 QI e also others, for instance, the Immerather and the 



