Mr Toulett Scrope's Considerations on Volcanos. 343 



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 depend the direction 

 taken by the lava, the velocity of its progress, the extent of its superfici- 

 al spread, and, consequently, the figure of the rock into which it con- 

 geals. 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 their imperfect fluidity, (owing 

 to a coarse grain and low specific 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 fivourable circumstances, the trachytic la- 

 vas 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 quaqua- 

 versal dip,) both of trachyte, (of the standard trachyte of the French geo- 

 logists) and of basalt, alternate with each other, and with interposed beds 

 of ashes, or volcanic 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 dis- 

 tant, 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 flu- 

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

 elastic vapour interposed between these would give a great mobility to the 

 mass, in the direction of their longest axes. The crystals of all lavas, in- 

 deed, are supposed by our author, when in motion, rather to slide or slip 

 past one another by means of the intervention of a small quantity of fluid 

 between their flat surfaces, than roll over one another, as is probably 

 the case with the globular particles of perfect fluids. This would natu- 

 rally result from their peculiar kind of fluidity, and also explains the ex- 

 treme difficulty with which lavas in motion are induced to swerve from 

 the direction they have once taken. The smallest obstacle is sufficient to 

 check their progress for some time, and even to consolidate the lava to 

 some distance back from the obstacle. These solidified parts, when again 

 broken up by the increased impetus of the lava behind, occasions the brec- 

 dated character of some lavas, where angular fragments are enveloped in a 

 paste of the same material. The zoned and ribboned structure of pearl- 

 stones is similarly accounted for. This sluggishness of lava currents oc- 

 casions great accumulations of the substance on those points where its 

 motion was checked and diverted, as in the angles of water courses, &c. ; 

 and examples are given from the Vivarais, where huge patches of colum- 

 nar basalt occupy the concave elbows of the gorges of the granite moun- 

 tains, the connecting strips being shallow, or having altogether disappear- 

 ed. The curious procedure of lava, when it meets with a perpendicular 



