250 Notice of Active and Extinct Volcanos. 



1st variety, glistening, base composed of compact felspar with an 

 enamelled surface, easily fusible. 



2nd, semi-vitreous (vitro-lithoide,) almost entirely composed of 

 semi-vitreous globules, amongst which are disseminated crys- 

 tals of glassy felspar, and some of quartz. 



3d, seoriform ; paste semi-vitreous and dull, porous, or with ir- 

 regular and imperfect cells. 



4th, cavernous ; paste scarcely discernible ; small and ver} r nu- 

 merous cells ; irregular cavities of various sizes ; mass infu- 

 sible. 



2d Subspecies, without quartz : base of compact felspar with or 

 without lustre, more or less fusible before the blowpipe; small 

 crystals more or less numerous, often with imperfect termina- 

 tions, of glassy or earthy feispar ; black mica in small hexa- 

 gonal plates ; no crystals of quartz or semi-vitreous globules. 



1st variety glistening; base of compact felspar, easily fusible 

 into a white enamel ; small crystals of felspar, commonly of 

 the glassy kind. 



2d dull ; base of compact felspar, dull, difficulty fusible before 

 the blowpipe ; small crystals of felspar, commonly earthy, 

 sometimes very rare. 



3d, cellular or pumiceous ; base almost infusible, full of cells ; 

 crystals of felspar rare and indistinct. 

 3d Species, Pearlstone. 



1st variety, testaceous, made up of an assemblage of vitreous glo- 

 bules more or less distinct, generally scaly (testaces) and with 

 a pearly lustre ; mica and felspar very rare. 



2nd, spherolitic paste of pearlstone not testaceous, with an ena- 

 melled lustre, and a grey color ; numerous crystals of very 

 brilliant black mica ; glassy felspar in small crystals, ordinarily 

 with their terminations imperfect. 



3d, pitchstone, vitreous paste approaching to obsidian, often with 

 a fatty lustre ; crystals of glassy felspar with imperfect ter- 

 minations ; little geodes of chalcedony more or less numerous. 



4th, globular stony, stony mass, composed of globules with a com- 

 pact or radiated structure, semi-vitreous or altogether stony. 



5th, stony in mass. The whole mass semi-vitreous, or altogether 

 stony ; the structure passing sometimes into porphyritic. 



The Millstone Porphyry is so uniform in its composition, as not 

 to admit of being distinguished in the manner of the preceding 

 species ; but 

 The 5th Species, Trachytic Conglomerate, is divided into 



1st, the conglomerates made up of the debris of Trachyte, cemen- 

 ted by an earthy or more or less crystalline paste. 



2d, the conglomerates consisting chiefly of the trachyte and millstone 

 porphyry, rounded or angular. 



