Ch. XXVIIL] 



LAVA — SCOKI^ — PUMICE. 



409 



are seen to have a glazed or vitreous coaticg, and in this respect exactly 

 resemble scoriaceous lavas, or the slags of furnaces. 



Fig. 62a 



Scoriaceous la-va in part converted into an 



amj'gdaloid. 



Montagn e de la Veiile, Department ot Pay 



de Dome, France. 



The annexed figure represents a 

 fragment of stone taken from the 

 upper part of a sheet of basaltic lava 

 m Auvergne. One lialf is scoria- 

 ceous, the pores being perfectly emp- 

 ty ; the other part is amygdaloidal, 

 the pores or cells being mostly filled 

 up with carbonate of lime, forming 

 white kernels. 



Lava. — This term has a some- 

 what vague signification, having 

 been applied to all melted matter 

 observed to flow in streams from 

 volcanic vents. When this matter 

 consolidates in the open air, the 

 upper part is usually scoriaceous, 

 and the mass becomes more and 



more stony as we descend, or in proportion as it has consolidated more 

 slowly and under greater pressure. At the bottom, however, of a stream 

 of lava, a small portion of scoriaceous rock very frequently occurs, formed 

 by the first thin sheet of liquid matter, which often precedes the main 

 current, or in consequence of the contact with water in or upon the 

 damp soil. 



The more compact lavas are often porphyritic, but even the scoriaceous 

 part sometimes contains imperfect crystals, which have been derived from 

 some older rocks, in which the crystals pre-existed, but were not melted, 

 as being more infusible in their nature. 



Although melted matter rising in a crater, and even that which 

 enters a rent on the side of a crater, is called lava, yet this term 

 belongs more properly to that which has flowed either in the open 

 air or on the bed of a lake or sea. If the same fluid has not reached 

 the surface, but has been merely injected into fissures below ground, it 

 is called trap. 



There is every variety of composition in lavas ; some are trachytic, as 

 in the Peak of Tenerifie ; a great number are basaltic, as in Vesuvius and 

 Auvergne ; others are Andesitic, as those of Chili ; some of the most 

 modern in Vesuvius consist of green augite, and many of those of Etna of 

 augite and Labrador-felspar.^ 



Sconce and Pumice may next be mentioned as porous rocks, pro- 

 duced by the action of gases on materials melted by volcanic heat. 

 Scoriae are usually of a reddish-brown and black color, and are the 

 cinders and slags of basaltic or augitic lavas. Pumice is a light, spongy, 

 fibrous substance, produced by the action of gases on trachytic and other 



• G. Rose, Ann. des Mines, tonu viii. p. 32. 



