STAFFA. GEOLOGY. 19 



island, consists of a black basaltic rock containing frag- 

 ments of the same substance, sometimes more compact 

 than the matter of the bed, at others amygdaloidal or 

 cavernous. Where the sea washes it, the fragments are 

 generally distinguishable, partly by their superior dura- 

 bility and consequent projection, and partly by their resist- 

 ing the adhesion of the common Lepas which covers the 

 other parts of the rock. 



The basalt which forms the substance of the columns, 

 is of a dark greyish black colour and of an uniform com- 

 pact texture, somewhat sonorous arid very brittle; breaking 

 with violence into sharp and thin fragments, or into irre- 

 gular angular masses. When bruised, it has an obscure 

 green colour, and the powder is of a muddy and greenish 

 white. It is difficult to ascertain truly its composition 

 by the magnifying glass, the particles of which it is 

 composed being extremely minute ; and it is sufficiently 

 evident that no mode of analysis, chemical or mechanical, 

 is applicable to an investigation of this nature. As far 

 as can be perceived by the lens, this rock seems to consist 

 of a large proportion of a dark grey substance mixed with 

 a smaller one of a black colour ; but it is impossible to 

 ascertain to which of the two the green colour of the 

 powder is owing, as the minutest scratch that can be made 

 involves both. It is not improbable that it is a minute 

 compound of augit with compact felspar ; an opinion some- 

 what countenanced by the peculiar green colour of the 

 powder, and by the great prevalence of this mineral among 

 the traps of the Western islands. Whatever dissimilitude 

 the basalt of Staffa may bear to those of Germany, or 

 to the well characterized examples of this substance which 

 occur in this country, it does not appear possible at present 

 to separate it from them by any mode of definition ; and 

 we must therefore be content to call it by this name until 

 we shall have acquired a more thorough acquaintance with 

 the several varieties of this multifarious family, and shall 

 have discovered a method of distinguishing and defining 

 them. I shall only add that it cannot be distinguished 



