TRUE VOLCANOES. 417 



midst of beds of granite and gneiss. A totally opposite con- 

 dition of stratification is exhibited in tlie volcanic district of 

 Eifel, as I have already observed, both from the activity 

 which once manifested itself in the Maars (or mine-funnels) 

 sunk in the Devonian schist, and that shown in the raised 

 structures from which lava streams flow, as on the long ridge 

 of the Mosenberg and Gerolstein. The surface does not here 

 indicate what is hidden in. the interior. The absence of tra- 

 chyte in volcanoes which were so active thousands of years 

 ago is a still more striking phenomenon. The augitiferous 

 scoriae of the Mosenberg, which partly accompany the ba- 

 saltic lava stream, contain small burned pieces of schist, but 

 no fragments of trachyte, and in the neighborhood the tra- 

 chytes are absent. This species of rock is only to be seen in. 

 the Eifel in' a state of entire isolation,* far from the Maars 

 and lava-yielding volcanoes, as in the Sellberg and Quiddel- 

 bach, and in the mountain chain of Keimcrath. The differ-- 

 ent nature of the formations through which the volcanoes 

 force their way, so as to operate with power on the outer 

 crust of the earth, is geologically as important as the mate- 

 rial which they throw out. 



The conditions of configuration in those rocky structures 

 through which volcanic action manifests itself, or has en- 

 deavored to do so, have at length been in modern times far 

 more completely investigated and described, in their often 



According to Boussingault, the ejected fragments of trachyte brought 

 home by Wisse, and collected on the upper descent of the cone (the 

 traveler reached an elevation of 960 feet below the summit, which is 

 itself 485 feet in diameter), consist of a black, pitch-like fundamental 

 mass, in which are imbedded crystals of glassy (?) feldspar. It is a 

 very remarkable phenomenon, and one which up to the present time 

 seems to stand alone in the history of volcanic ejections, that, along 

 with these large black pieces of trachyte, small sharp-edged fragments 

 of pure quartz are thrown out. According to a letter from my friend 

 Boussingault, dated January, 1851, these fragments are no longer than 

 four cubic centimetres in bulk. No quartz is found disseminated in 

 the trachytic mass itself. All the volcanic trachytes which I have ex- 

 amined in the Cordilleras of South America and Mexico, and even the 

 trachytic porphyries in which the rich silver veins of Real del Monte, 

 Moran, and liegla are contained, to the north of the elevated valley 

 of Mexico, arc entirely destitute of quartz. Notwithstanding this seem- 

 ing antagonism, however, of quartz and trachyte in still-active volca- 

 noes, I am by no means inclined to deny the volcanic origin of the 

 " trachytes et porphyrcs meulieres (mill-stone trachytes)" to which Beu- 

 dant first drew attention. The mode, however, in which these arc 

 formed, being erupted from fissures, is entirely different from the form- 

 ation of the* conical and dome-like trachyte structures. 



* See above, p. 321-225. 



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