122 Volcanos. 



of fluidity inversely proportioned to the average size of their 

 crystalline particles, or grain. And, vice versa, when their grain 

 is of the same degree of fineness, their fluidity will be propor- 

 tioned at their specific gravity, i. e. to the proportion of ferru- 

 ginous minerals in their composition. 



" The author illustrates these propositions, by showing, from 

 observation, that the basaltic lavas have generally flowed far- 

 ther, and spread over a larger surface than the trachytic; and 

 also that the lavas of either class have spread in a horizontal di- 

 rection, more or less in proportion to the adundance of the heav- 

 ier minerals in their composition, and the fineness of their grain. 

 The fact has been long ago remarked, but the explanation of 

 it, is presumed to be novel. The disposition of a body of lava, 

 emitted from an orifice in the surface of the earth, is in strict- 

 ness, determined by, 1. The force of expulsion — 2. Its fluidity — 

 3. The external circumstances that may render this fluidity more 

 or less permanent — 4. Those which favour or impede the lateral 

 extension to which it is urged by its fluidity. On. the compound 

 influence of these circumstances depends the direction taken by 

 the lava, the velocity of its progress, the extent of its superficial 

 spread, and consequently, the figure of the rock into which it 

 congeals. According to these, lavas assume the form either of 

 sheets, streams, hummocks, or domes. Examples are given, in 

 numbers, of their modifications of form; and it is observed, that 

 the general bulkiness of the trachytes is simply accounted for by 

 by their imperfect fluidity, (owing to a coarse grain and low spe- 

 cific gravity) without presuming that they have swelled up like 

 a bladder from below, according to the vague and anomalous idea 

 of Humboldt and De Buch. The author proves, from his own 

 observations however, in opposition to the statements of Beudant 

 and other writers, that, under favorable circumstances, the trachy- 

 tic lavas have often spread into bulky sheets and streams, (nappes 

 et couleis) particularly in the Mont Dor ; from which he gives a 

 section where beds (slightly inclined away from the centre of the 

 mountain with a quaquaversal dip,) both of trachyte, (of the 

 standard tratchyte of the French geologists) and of basalt, alter- 

 nate w T ith each other, and with interposed beds of ashes, or vol- 

 canic conglomerate. The author mentions one vast stream of 

 feldspathose lava (clinkstone) which appears to have flowed 

 from the summit of" the Mezen, in Velai, into the bed of the 

 Loire, thirty miles distant, with an average width of six miles, 

 and a thickness of 500 feet ; thus rivalling the colossal trachytes 

 of the Andes. Clinkstone, or the laminar variety of trachyte, is 

 presumed to have possessed in general a superior fluidity, owing 

 to the parallelism of its crystals, as a very small proportion of 

 elastic vapour interposed between these would give a great mo- 



