Ch. XXXI.] 



TERTIARY VOLCANIC ROCKS. 



541 



sandstones and shales, with some associated limestones, replete with fossils 

 of the Devonian or Old Red Sandstone group. The volcanos broke out 

 in the midst of these inclined strata, and when the present systems of 

 hills and valle) T s had already been formed. The eruptions occurred 

 sometimes at the bottom of deep valleys, sometimes on the summit of 

 hills, and frequently on intervening platforms. In travelling through 

 this district we often fall upon them most unexpectedly, and find ourselves 

 on the very edge of a crater before we had been led to suspect that we 

 were approaching the site of any igneous outburst. Thus, for example, 

 on arriving at the village of Gemund, immediately south of Daun, we 

 leave the stream, which flows at the bottom of a deep valley in which 

 strata of sandstone and shale crop out. We then climb a steep hill, on 

 the surface of which we see the edges of the same strata dipping inwards 

 towards the mountain. When we have ascended to a considerable height, 

 we see fragments of scorise sparingly scattered over the surface ; until, at 

 length, on reaching the summit, we find ourselves suddenly on the ecge 

 of a tarn, or deep circular lake-basin (see fig. 673). 



Fig. 678. 



The Gemunder Maar. 



Fisf. 6T4. 



a. Village of Gemund. 



b. Gemunder Maar. 



c. Weinfeldcr Maar. 



d. Schalkenmchren Maar. 



This, which is called the Gemunder Maar, is one of three lakes which 

 are in immediate contact, the same ridge forming the barrier of two 

 neighboring cavities. On viewing the first of these (fig. 673), we recog- 

 nize the ordinary form of a crater, for which we have been prepared by 

 the occurrence of scorise scattered over the surface of the soil. But on 

 examining the walls of the crater, we find precipices of sandstone and 



