. [ 104 ] [Book VL 



CHAP. XVH. 



VOLCANIC PRODUCTS. 



, Different Kinds. -Progrefs of a River of Lava Confound 

 Parts of Lava. Pumice Stone. Be.Jaltes.~- Trapp. Terra Puz- 

 zolana. 



AVA is of very various appearance, according 

 to its compofition, and its more or lefs per- 

 fect vitrification. The materials of which lavas con* 

 fift, are the common fubftances to be found every 

 where in the earth, namely, flones, metallic ores, clay, 

 fand, &c. -, and as there is room for great variety in 

 the combinations of thefe fubftances, the melted mafles 

 formed by them mud in different circumftances be 

 very various. Some lavas are very compact, and re- 

 ceive a beautiful polifh, exhibiting great variety of 

 colours and forms. Others are extremely porous, 

 and mixed with fcoriae or drofs. On the different 

 confidence of lavas depends their capacity for being 

 converted into mould, capable of fupporting vegeta- 

 bles. Some lavas have a tendency to crumble into 

 duft immediately on iffuing from the crater; others are 

 a perfect glafs, and are calculated to refift for a long 

 time every approach towards duTolution. 



Lava, when firft thrown out from the crater of a 

 volcano, is an imperfect liquid mafs, and flows down 

 the fide of the mountain with confiderable rapidity. 

 From lofs of heat, however, its furface is foon con- 

 verted into a tough and black cruft, which, as it be- 

 comes thicker, gradually impedes the progrefs of the 

 fluid lava which is contained within it. This cruft is 

 frequently broken, when the ftream of lava refembles 



a river 



