158 OBSERVATIONS ON 



has exhibited only the incipient appearance of regular 

 forms ; and doul)tless every rock of the kind would, if pe- 

 netrated to a sufficient depth, be found to be a solid mass, 

 Humboldt, it is true, in his account of the breaking; out of 

 the volcano of Jorullo in the intendancy of Valladolid in 

 Mexico, says that strata of clay, enveloping l)alls of decom- 

 posed l)asalt in concentric laijers, were thrown out : but 

 these, although actually ejected in a figurate shape, it clearly 

 appears, were not then recently formed, but were the pro- 

 duct of anterior convulsions, and had long lain in the ground 

 in a state of decay. But the crystalline form of basalt has, 

 at all events, received an importance with respect to the 

 question of origin, which it by no means deserves : for al- 

 though it is conceded that columnar and other figurate forms 

 exist in rocks of aqueous origin, it has been proved, by the 

 well known experiments of Mr, Gregory Watt, that both 

 the columnar and globular structure may be produced by 

 the slow refrigeration of a mass of melted basalt ; and lava 

 has been found in Filicuda, one of the Lepari islands, in 

 perfect columns, and imperfectly columnar in the island of 

 Ponga. 



The second species is soft and friable throughout, usually 

 amorphous, and of every intermediate shade between a dark 

 and an ash grey ; but most frequently of the latter. It is 

 of a coarse open structure, and so soft, that some specimens 

 may be crumbled between the fingers. It is not incrustated ; 

 but the whole mass, when disintegrated, falls into a coarse 

 sand with a rather slight intermixture of clay. It is not 

 very abundant, and is used witii advantage in giving a 

 smooth surface to tlie turnpike road which leads from Lan- 

 caster to Harrisburg ; the body of which is constructed of 

 the harder species, which is also sometimes amorphous. 



Both kinds, wiien exposed to cwistant moisture, are co- 

 vered witii a reddish brown, whose depth of colour is in 

 proportion to their hardness : hence the harder masses con- 

 tain more iron. These rocks correspond, in almost every 

 particular, with the descriptions we have of the incrustated 



