^6 Mr. PouLETT ScROPE on the Geology of the Ponza Isles. 



below upwards, through the broken and separated portions of overlying rocks 

 that opposed its expansion. 



In all these cases the curvatures and contortions of the stripes are obviously 

 owing- to the obstacles, whether internal or from without, by which the uniform 

 progress of the liquid matter was impeded ; and principally to the various 

 degrees of mobility of its different layers, those of coarser grain (and particu- 

 larly the spherolitic concretions,) retarding the motion of the proximate layers 

 which possessed a greater liquidity. The same remark applies with equal 

 force to the similar rocks of the American Cordilleras. These are described 

 by M. de Humboldt as uniformly ribboned, and often to such a degree as to 

 put on the appearance of stratification ; as exhibiting a base composed of 

 bluish-gray semi-vitreous globules, closely compacted together, and inclosing 

 parallel zones and lenticular masses of obsidian (Cinapecuaro, Oio del Agua, 

 del Pinal, &c. *); as enveloping fragments of reddish brown trachyte, half- 

 fused, at one extremity of parallel elongated cellular cavities. It is obvious 

 at once how closely these characters correspond to those of the pearlstone 

 of Ponza and Palmarola ; and they are explained with the utmost facility by 

 the process which is proved to have given rise to them in the latter case. 

 The last variety of perlite quoted above affords a beautiful illustration of the 

 truth of this theory. It is quite evident that the elongated cells which stretch 

 in parallel lines behind half-fused fragments of felspar (spherolites, in the 

 words of M. de Humboldt, ''like the tail of a comet from its nucleus,") were 

 created by the inferior mobility of these coarse fragments to that of the finer 

 enveloping mass, and are analogous to the hollow that forms behind any 

 stationary or resisting body which is plunged into a running stream. These 

 cells therefore are certain indications of the direction in which the mass was 

 moving immediately before its consolidation. 



I may remark that the pearlstone of Oyamel (the Mountain of Knives) in 

 Mexico, veined with obsidian, bears a still further analogy to that of Palma- 

 rola in the vertical direction of its zones, by which these masses are proved 

 to have been propelled from below in the manner of dykes. 



It may be mentioned that some varieties of the porphj/re molaire of M. 

 Beudant, as far as can be distinguished from the specimens I have had the 

 opportunity of seeing, are identical with the siliceous parts of the prismatic 

 trachyte of Ponza. The similar varieties that occur in the island of Ischia 

 have been already adverted to. 



The siliceous trachyte of the Ponza group must be conceived to correspond 



* Essai Geognostjque, «S:c. pp. 341, 342. 



