PHYSIOGRAPHY. CLASS II. 



nular, strongly connected, so as to be scarcely re- 

 cognizable ; fracture more or less perfectly conchoi- 

 dal, uneven and splintery. The whole mass is 

 often traversed with separating faces, which may 

 be considered as rudiments of the faces of lamellar 

 composition : often the composition is granular, 

 thick or thin, and generally bent ; the faces of 

 composition being smooth, and possessing pearly 

 lustre. Small grains of Obsidian are often enve- 

 loped in a number of successive thin coats, several 

 of these again are surrounded by other coats, and 

 so on several times, which produces a very remark- 

 able composition. Vesicular; the cavities often 

 elongated in one direction, parallel and in such 

 number, that the mass appears fibrous and of a 

 pearly lustre. 



OBSERVATIONS. 



1. The varieties of empyrodox Quartz are intimately 

 connected by transitions. These transitions are particu- 

 larly important in the determination of the species in Na- 

 tural History, when there are no regular forms or faces of 

 cleavage. Yet they require to be carefully treated, and, as 

 in the present case, it is necessary to give proper attention 

 to hardness and specific gravity. Though in most minera- 

 logical systems the four species of Obsidian, Pitchstone, 

 Pearlstone, and Pumice, which must be comprised within 

 that of empyrodox Quartz, are placed immediately follow- 

 ing each other, yet they are not brought into that con- 

 nexion which evidently takes place in nature, if we consi- 

 der their properties. As an empirical demonstration of 

 the correctness of this view, we may take the difficulty of 

 an accurate distinction of these species. Obsidian possesses 

 the most perfect conchoidal fracture,] and high degrees of a 



