ON ITS EXTERIOR. VOLCANOES. 425 



having no lava-streams (Charles Deville considers the 

 equally active volcano of Stromboli to be similarly with- 

 out such), but which, at least since the year 1728, has 

 been in constant activity, erupting black and often 

 brightly-glowing rocks and stones, forms an island of 

 trachyte, of scarcely so much as 8 miles diameter ( 586 ), 

 in the midst of granite and gneiss. Very different rela- 

 tions of position are presented, as I have already re- 

 marked, in the volcanic district of the Eifel, in the Maars 

 (mine-funnels) sunk in the Devonian schists, as well as 

 in the lava-yielding volcanic frameworks of the long 

 ridges of the Mosenberg and Grerolstein. The surface 

 does not here betray what is concealed within. The 

 absence of trachyte, in volcanoes which were so active 

 many thousand years ago, is a still more striking phe- 

 nomenon. The augite-containing scoriae of the Mosen- 

 berg, which in part accompany the basaltic lava-stream, 

 have embedded in them small burnt pieces of schist, 

 not fragments of trachyte; there are no trachytes in 

 the vicinity. This last-named rock is only visible in 

 the Eifel district ( 587 ), quite apart and at a distance 

 from Maars and lava-yielding volcanoes, as in the Sell- 

 berg, near Quiddelbach, and in the ridge of Eeimerath. 

 The variety of formations broken through by volcanoes, 

 in rising through the upper crust of the earth, are 

 geologically no less important than are the substances 

 which they produce. 



In the most distant parts of the earth the forms of 

 the rocky frameworks through which the volcanic acti- 

 vity manifests itself, or has striven to do so, liave 

 been far more carefully examined and represented in 

 their often very complicated diversity, than was the 



