680 



TERTIARY VOLCANIC ROCKS. 



[Ch. XXXI. 



formed. The eruptions occurred sometimes at the bottom of deep 

 valleys, sometimes on the summit of hills, and frequently on inter- 

 vening platforms. In travelling through this district we often fall 

 upon them most unexpectedly, and may find ourselves on the very 

 edge of a crater before, we had been led to suspect that we were ap- 

 proaching 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 con- 

 siderable height, we see fragments of scoriae sparingly scattered over 

 the surface ; until, at length, on reaching the summit, we find ourselves 

 suddenly on the edge of a tarn, or deep circular lake-basin (see fig. 

 125). 



Kg. 725. 



The Gemunder 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. 725), 

 we recognize 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 shale which exhibit no signs of the action of heat ; 

 and we look in vain for those beds of lava and scorise, dipping out- 

 wards on every side, which we have been accustomed to consider as 

 characteristic of volcanic vents. As we proceed, however, to the 

 opposite side of the lake, and afterwards visit the craters c and d (fig. 

 726), we find a considerable quantity of scorise and some lava, and see 







Fig. 72G. 











(S^N. t ~ 



^*^^%> 







1 



K. & / 



x^S? 





^s~ 



£ 4 > /N 



^ 



^K~^~^ 







\ 















a, Village of Gemund. 

 5. Gemunder Maar. 



c. Weinfelder Maar. 



d. Schalkenmehren Maar. 



