OF THE WESTERN ISLES OF SCOTLAND. 37 



green, and the brown tint is the result of alteration. Only the 

 faintest trace of pleochroism can be detected. Some of the crystals 

 show the beginning of the development of a structure like that 

 exhibited by the augites in the older andesites of the district, w^hich 

 is described at page 363. Osann has described and analyzed a 

 very similar augite from the augite-andesite of Kolter, in the^Faroe 

 Islands *. Its composition is as follows : — 



Silica 50-21 



Alumina 3*24 



Perrons oxide 17*40 



Lime 13-92 



Magnesia 14-05 



98-82 



Rhombic pyroxene occurs in these rocks, but in small quantities, 

 and it must be regarded as an accessory constituent. 



3. Magnetite occurs in distinct individuals, as skeleton-crystals, 

 or in roundish grains, and is in some cases remarkably abundant. 



Olivine, like enstatite, is an accessory constituent and is very 

 variable in quantity. In some varieties of the rock it is not 

 rare, while in most cases it is wholly wanting. 



Brown glass containing microlites of different minerals and often 

 corroding and forming enclosures in the felspars is usually present. 

 Sometimes, when it is present in considerable quantities, this glass 

 exhibits traces of both the spherulitic and the perlitic structure. 



The best way of illustrating the remarkable varieties of struc- 

 ture and chemical composition which can be produced by combining 

 tlie same mineralogical constituents in varying proportions, nnd 

 with modification of internal arrangement, will be to describe some 

 of the leading types of the Ben Hiant rocks, and show how they 

 pass into one another by insensible gradations. 



1. Tijpical Pyroxene-andesites. — These consist of a more or less 

 perfectly glassy ground-mass crowded with microlites of felspar and 

 augite, and grains of augite, the whole forming a " microlitic felt." 

 Througli this base are scattered crystals of plagioclase, usually 

 abounding with glass-inclusions, and with remarkable zoned struc- 

 ture. Pyroxene is represented by both augite and enstatite, the 

 former being always the most abundant, though the latter mineral 

 is sometimes by no means rare. Magnetite grains also occur 

 scattered througli the base (see Plate XY. fig. 2). 



This rock not unfrequently contains vesicular cavities, Avhich are 

 usually filled with concentric deposits of various secondary minerals. 

 The whole rock presents the most remarkable analogy with some of 

 tlie well-known pyroxene-andesites of Hungary. Indeed, if some of 

 my Ben Hiant and Hungarian sections were accidentally mixed, 

 I know of no characters by which I should be able to separate 

 them. 



* Neues Jabrb. liir Min. (1884), i. p. 48. 



