316 J. W. JFDD ON THE ANCIENT VOLCANO OF 



after an interval of quiescence, by the final outbursts, first of a highly- 

 acid, and then of an extremely basic character. Have we here an 

 indication that during a period of temporary rest in a volcano, the un- 

 ejected materials below it may undergo a separation into two 

 " magmas " of different composition and specific gravity, similar to 

 those which, according to the well-known theory of Durocher, have 

 been universally formed below the earth's crust ? 



The formation of the scattered basaltic cones within the great 

 crater-ring of the Schemnitz volcano, and of a few similar ones 

 (" puys ") around its flanks, were the last great efforts of the igneous 

 forces of the district. Subsequently their declining energies appear to 

 have been equal only to the origination of fumaroles and hot springs or 

 to the production of those earthquakes to which the region is still occa- 

 sionally liable. The extent of the deposits, both siliceous ("sinter") 

 and calcareous ("travertine"), formed by these springs shows clearly 

 how long, even after the final extinction of the volcano, the subter- 

 ranean forces retained a portion of their vitality, and how very slowly 

 and gradually the igneous activity of this region sank into complete 

 quiescence. 



We have endeavoured in the preceding pages to investigate the 

 several varieties of rocks of which the old Schemnitz volcano was 

 built up, and, by the aid of the analogies of more perfect and still 

 active volcanos, to reconstruct the history of the events to which its 

 original features and its present condition are to be ascribed. Let 

 us now proceed to inquire what are the appearances exhibited in the 

 interior of this old volcanic pile as now exposed to us, which serve 

 to illustrate the nature of the operations going on deep below the 

 surface, synchronously with those phenomena which we ordinarily 

 witness at the surface as the accompaniments of volcanic activity. 



And although we have not here the remarkably clear sections af- 

 forded by sea-cliffs, as in the ancient Hebridean volcanos, yet in the 

 great volcano of Schemnitz — the whole heart of which has been blown 

 out by tremendous paroxysms, while the vast crater thus originated 

 has been further enlarged by the action of the sea which entered it 

 after the partial submergence of the mass — many valuable opportu- 

 nities are afforded to the geologist for studying the relations of the 

 originally deeply seated masses of the volcanic pile. Since the final 

 upheaval of the district, too, the great rock-masses have been greatly 

 reduced by subaerial denudation, and important sections of them ex- 

 posed in the valleys ; and, finally, the innumerable mining-works 

 opened all over the central area, and a series of deep railway-cut- 

 tings have all served to throw light upon the relations which the 

 different rocks of the district bear to one another. Aided by all 

 these different kinds of exposures of the rock-masses of the Schem- 

 nitz area, Ave propose to describe the structure of the interior of this 

 old volcano and to investigate the connexion between its internal 

 mechanism and the operations which we have shown to have taken 

 place at the surface. 



We have already exposed the grounds on which we have been led 

 to the conclusion that the so-called " granite and svenite," the 



