Notice of Active and Extinct Volcanos. 247 



are destitute of lustre, and exhibit at the same time sundry al- 

 terations in their size, structure, and mode of aggregation, till 

 at length they entirely disappear, and the whole mass puts on a 

 stony appearance, which retains none of the characters of pearl- 

 stone. On the other hand the globules, becoming less distinct 

 either resolve themselves into a paste resembling enamel, very 

 fragile, in which separate portions approaching to a spherical 

 form are indistinctly visible, or into a more vitreous and more 

 homogeneous mass, which is generally black, and presents all the 

 characters of pitchstone or obsidian. Among these latter vari- 

 eties is one which resembles the marekanite of Kamschatka. 



" Sometimes globules consisting of felspar occur in the rock, 

 which are either compact or striated from the centre to the cir- 

 cumference, and these are sometimes so numerous that the whole 

 mass is composed of them. Various alternations occur between 

 the glassy and stony varieties of the pearlstone, sometimes so 

 frequent as to give a veined or ribboned appearance to the rock, 

 at others curiously contorted as though they had been disturbed 

 in the act of cooling. 



" Lastly, all these varieties occasionally present a cellular, 

 porous, spongy, and fibrous aspect, and pass into pumice. With 

 respect to their chemical characters, it may be sufficient to re- 

 mark that the vitreous varieties of pearlstone usually effervesce 

 under the blow-pipe, but the stony do not. These rocks often 

 contain geodes of chalcedony and opal, the former existing in 

 the more vitreous, the latter in the more stony or felspathic por- 

 tions. The opal is commonly opaque, but is occasionally met 

 with more or less translucid. 



" The fourth species is distinguished for its hardness and cel- 

 lularity, qualities which have caused it to be employed all over 

 Hungary for the purpose of millstones, from whence the name 

 of Millstone Trachyte has been applied to it by Beudant. 



" Unlike the other rocks comprised under the same generic 

 term, it abounds in quartz, or in silex under some one of its 

 modifications, and in proportion as the latter earth is more or less 

 abundant, the substance puts on the characters either of horn- 

 stone or of clay porphyry. The paste is always dull and coarse 

 looking, its colors vary from brick-red to greenish-yellow, its 

 fracture is generally earthy, its hardness very variable, but usu- 

 ally considerable. It contains crystals of quartz, of felspar, 

 lamellar, and sometimes glassy, and of black mica, imbedded. 

 Jasper and hornstone also occur in nests, or in small contempo- 

 raneous veins very abundantly disseminated, and siliceous infil- 

 trations, posterior to the formation of the rock, seem likewise t© 

 occur among the cells which are every where distributed. 



" In examining these rocks with a glass, we discover a multi- 



