40 Descriptive Arrangement of Volcanic Rocks. 



as convenience may dictate. A number of appellations liavo 

 indeed been given, and confirmed by general use, to partic- 

 ular varieties; some of which it may be as well to specify in 

 this place. Thus the granular, massive, and earthy sub- 

 species of trachyte has been called in England claijstone ; 

 in France, domite ; necrolite., by Brocchi. Compact and 

 close-grained trachyte has received the name of compact fel- 

 spar, and perhaps of hornstone ; the laminar sub-species of 

 the same rock, clinkstone, (phonolite ;) and this name being 

 appropriated to a peculiarity of texture, is given as well to 

 greystone and basalt, as to trachyte, when possessed of that 

 character. 



Resinous trachyte is generally known by the name of pitch- 

 stone ; vitreous by that of obsidian ; and the same when 

 formed into globular concretions, perlite or pearlstone ; and 

 the same vitreous basalt has been called gallinace by the 

 French geologists. 



Spumous and filamentous trachytes are called pumice. 

 Spumous greystone and basalt, scoria. 



When porphyritic, many of these varieties have been call- 

 ed porphyries ; as claystone porphyry, pitchstone porphyry, 

 &c. Large grained or granitic basalt has the title o{ green- 

 stone (dolerite.) Very coarse grained trachytes have, per- 

 haps, often been described as syenites. 



With regard to the conglomerates, the sandy and fine- 

 grained varieties of trachytic conglomerate are generally call- 

 ed tufa, sometimes trass ; the coarse and incoherent, lapillo. 

 Those of greystone also usually bear the same appellations. 

 The basaltic conglomerates are occasionally styled peperino, 

 or trap-tuff; but when fine, or sandy and incoherent, puzzo- 

 lana. Some fine tufas, indurated by water, and, with a la- 

 mellar structure, have been made use of and described as 

 tripoh. Basaltic conglomerates, when much decomposed, 

 have been designated by the term wacke, as well as the con- 

 generous lithoidal rock when in a similar condition. 



