200 Mr. PouLETT ScROPE on the Geology of the Ponza Isles. 



in character. The mineral composition of these varieties is very similar. All 

 consist principally of felspar, and differ only in a greater or less admixture of 

 mica and quartz. 



The rock, which from its superior durability appears to support and 

 strengthen the less solid parts with which it is intimately associated, is univer- 

 sally prismatic, and generally divided into groups of small but very regular 

 columns, from which common character I will distinguish it for the present 

 by the name of prismatic trachyte. Its predominant colour when broken is 

 grayish or whitish yellow, mottled with irregularly parallel streaks of a pinkish 



cent. Its specific gravity does not exceed 2*80. It is fusible before the blowpipe into a white 

 or grayish white enamel. Its other occasional ingredients are hornblende, augite, mica, titaniferous 

 iron, magnetic iron, and quartz. One or more of these minerals appears to be always present. 



The texture of trachyte varies from coarsely crystalline or granitoidal, to earthy, fine-grained, 

 scaly, and compact; and finally to resinous and glassy (obsidian). 



Sometimes the grains ex minute crystals of which it is composed are of uniform size, and the 

 structure of the rock is simple. In general, however, the size of these elements is very unequal, 

 and it assumes a porphyritic structure ; the larger crystals distinguishing themselves from the 

 base or enveloping matter of finer grain. 



These distinct crystals consist of some or all of the different minerals enumerated above as en- 

 tering into the composition of the rock. 



Those of felspar are frequently striated and even fibrous. The mica appears usually as hexa. 

 gonal or rhomboidal plates or prisms of great regularity. Hornblende shows itself in acicular 

 crystals. Titaniferous iron in octahedrons or minute grains. Quartz occasionally in rounded 

 grains, more rarely in hexahedral prisms terminated pyramidally at one or both extremities. 



The internal structure of trachyte embraces the following modifications : viz. 



The compact, porous, cellular, vesicular, cavernous, and filamentous. 



In its divisionary structure it presents occasionally the massive, columnar, prismatic, tabular, 

 laminar, schistose, globular, or angulo-globular forms. 



Its characters of lustre and fracture will be of course decided by the texture, those of hardness 

 by the texture and mineral composition. 



N.B. The granular and earthy varieties of this rock have by some geologists been called clay, 

 stone. The scaly and laminar, cA'wA:s/one. T\\e com'pzct, compact felspar^ ov hornstone. The 

 T^smows^pitchstone ; and the vitreous, obsidian. The vesicular and filamentous varieties of glassy 

 trachyte pass currently under the name oi pumice, and the globular concretionary variety, under 

 that of pearlstone. 



These names, which it will be well to retain, must not yet prevent our recollecting the rocks to be 

 all mere varieties of the genus /racA^yfe, characterizedbydifferences of texture and internal structure. 



I use the term trachyte in preference to that of claystone, the English synonyme for its most 

 ordinary variety, since it leads into no error as to the nature of the rock, which being in general 

 more or less crystalline, and containing occasionally above 90 per cent of silex, has little in com- 

 mon with clay. Trachyte appears to be universally of volcanic origin. It occurs in the volcanic 

 districts of France, Italy, the Rhine, Asia Minor, and the Archipelago : in the Azores, Leeward 

 Isles, and the Andes of both Americas. 



