§ 9. LAVA CURRENTS, DIKES, AND VEINS. ol I 



structure is slaty, Clinkstone. The same substance forms 

 augite when it cools rapidly, and hornblende when the 

 refrigeration takes place slowly. 



The lavas ejected from Vesuvius present considerable 

 variety of appearance and composition : they occur in the 

 state of pumice-stone ; vesicular scoriae, that is, cinders full 

 of hollow cells ; and compact heavy masses of molten rock, 

 which are sometimes spotted internally with red, yellow, 

 and grey. Mica occurs plentifully in some recent trachytes, 

 but crystallized quartz and hornblende, so abundant in 

 granite, are extremely rare. Pumice is supposed to be 

 produced by a considerable disengagement of vapour having 

 taken place while the lava was in a plastic, but not entirely 

 in a fluid state ; the escape of the gaseous matter giving 

 rise to the porous structure of this mineral. Dolomieu 

 observes, that one kind of pumice seems to be derived from 

 the fusion of granite, since it contains fragments of quartz, 

 mica, and felspar, and that when such fragments were ex- 

 posed to heat they were converted into a substance resem- 

 bling the surrounding pumice. 



9. Lava currents, dikes, and veins. — As the aspect 

 and nature of lava currents will be easily comprehended, 

 by the descriptions of volcanic eruptions which I shall 

 presently place before you, it will suffice to mention in this 

 place, that the appearance of lava in motion is that of a 

 sluggish viscid stream loaded with red-hot cinders and 

 ashes, and detached fragments of rocks, rolling one over the 

 other, and producing a loud crackling noise. Captain 

 Basil Hall aptly compares the movements of a lava-current 

 to that of a glacier ; — u They are both," he observes, 

 " more or less, frozen or half-congealed rivers ; they both 

 obey the law of gravitation with great reluctance, being 

 essentially so sluggish, that although they move along the 

 bottoms of valleys with a force well-nigh irresistible, yet 

 their motion is sometimes scarcely perceptible. Both glaciers 



