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PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



[Feb. 2, 



observed phenomenon of the kind. The small hummocks or mame- 

 lons of glassy felspathic lava upon the summit of the volcano of 

 Bourbon, which Bory de St. Yincent watched and drew, in actual 

 process of formation by the welling-up of an enduring source of 

 highly viscous matter at a white heat, which consolidated as it 

 trickled down the slope of the hill in concentric coatings, are, m. all 

 probability, types of the mode of production of all the larger trachytic 

 bosses. (See figs. 14 and 15.) In the galleries excavated by the 

 Eomans in the flanks of the Puy de Sarcouy in Auvergne, I observed 

 indications of such a structure in concentric coats Hke those of an 

 onion. (See fig. 16.) 



The cones formed by the mud-volcanos of Macaluba in Sicily, and 

 of Bella near the Indus, are apt illustrations, as has been already 

 suggested, of the probable mode of production of the bell-shaped 

 domes of trachyte. They are, of course, solid unless where a crater 

 has been left on the summit, but must, from the way in which they 

 have been formed by the overflow of one coat of mud over another, 

 be composed of concentric though irregular quaquaversal beds or 

 layers. The accompanying woodcut (fig. 17), copied from Bory de 

 St. Vincent, of the summit of one of the Mamelons of Bourbon erupt- 

 ing its viscous and glassy lava, well exhibits this analogy. 



Fig. 17. — Summit of one of the Mamelons of Bourbon, in eruption. 

 (After Bory de St. Vincent.) 



Where lavas of this imperfect liquidity were emitted simulta- 

 neously from several contiguous orifices on the same fissure, the re- 

 sulting hummocks will have been compounded into a bulky ridge 

 with more or less of an anticlinal structure, or a string of domes, 

 such as are not unfrequently observable in trachj'tic- formations*. 



* On reference to my volume on ' Volcanos ' (1825), it will be seen that at that 

 date I published this same view almost totidem verbis (p. 96), which M. de Hum- 

 boldt has since appeared inclined to adopt in reference to the great chains of 

 tapachyte in the Cordilleras (Kosmos, iv. pp. 289 & 307, English ed. 1858), — a 

 view, however, which is evidently quite inconsistent with the theory of upheaval, 

 as I had remarked in p, 93 of the same volume. 



