ON ITS EXTEUIOR. YOLC ANDES. 231 



" The Maars," as is acutely remarked by Captain von 

 Dechen, " seem by their formation as if they belonged to 

 nearly the same epoch as streams of lava from true 

 volcanoes. Both are situated near deeply cut valleys. 

 The lava-yielding volcanoes were certainly active at a 

 period when the valleys had already received very 

 nearly their present form ; and we actually see the 

 oldest lava-streams of the district falling precipitously 

 into the valleys." The Maars are surrounded by frag- 

 ments of Devonian schist and of heaped-up grey sand 

 and tufa margins. The Laacher See, whether we 

 regard it as a great Maar, or as it is regarded by my 

 friend C. von Oeynhausen as part of a great caldron- 

 valley in the clay-slate, shows on its surrounding 

 borders some volcanic scoriae ; as at the Krufter Ofen, at 

 the Veitskopf and the Laacher Kopf. But it is not only 

 by the entire absence of streams of lava (as they are 

 observed in the Canary Islands, at the outer margin 

 of true craters of elevation, or in their immediate 

 neighbourhood), or by the inconsiderable height of 

 their margin, that the Maars are distinguished from 

 craters of elevation ; their borders are also wanting 

 in that regular, always convex, outward slope in the 

 stratification, which is a consequence of upheaval. As 

 has been already remarked, the Maar depressions in 

 the Devonian schists appear like mine-funnels into 

 which, immediately after the violent explosion of hot 

 gases and vapours, the greater part of the light loose 

 masses emitted (Rapilli) again fell back. I name here, as 

 examples, only the Immerath Maar and those of Pulver 

 and Meerfeld. In the midst of the first, of which the 

 dry floor at two hundred feet deep is cultivated, are 



