OF THE WESTEKX ISLES OF SCOTLAND. 363 



consists essentially of a mesli of oligoclase and perhaps of anortho- 

 clase microlites entangling granules of augite (with some enstatite) 

 and of magnetite, glass being present in small quantities only and 

 quite inconspicuous. 



This Stiklcisliolmur type is well represented at many points in 

 the Western Isles of Scotland. Good illustrations of the type occur 

 in the promontory stretching out to Salen Pier in Mull, at Beinn 

 TJaig, and Creagach Beinn, in the same island, in Ardnamurchau, and 

 in many other places. 



Like the Icelandic rocks, those of our Western Isles are " com- 

 pact and dark-coloured . . . the fracture is often nearly conchoidal, 

 and certain specimens present a semi-vitreous appearance " (Bre'on, 

 loc. cit. p. 23). 



In his reference of these rocks of the StihJcisliolmur type to the 

 augite-andesites, Bre'on is supported by Eosenbusch, who, in the 

 last edition of his ' Massigo Gesteine ' (p. 682), refers to these rocks 

 as presenting some analogies with the augite-andesites described 

 by Eoerstner as occurring in Pantellaria, and containing anortho- 

 clase and the triclinic amphibole — cossyrite (aenigmatite). 



Some of the augite-andesites of the StiJchisliolmur type contain 

 large scattered crystals of anorthito or labradorite, and thus pass 

 into the labradorite-andesites. 



Another variety of the " trachytoid " pyroxene-andesites is pre- 

 sented when, in addition to the porphyritic crystals of felspar, large 

 phenocrysts of augite make their appearance. These augites appear 

 to belong to a variety rich in iron and magnesia, but which, con- 

 sidering their composition, are of remarkably stable character, often 

 remaining comparatively unaltered when all the crystals in the 

 rock have been profoundly changed. Such porphyritic augites often 

 show a tendency to assume the form of stellar aggregates, and 

 sometimes are of such dimensions as to be quite conspicuous on the 

 fractured surfaces of the rocks. Beautiful examples of lavas of this 

 type are found about Mingary Castle in Ardnamurchau, and I have 

 also detected them at many other points in the Western Isles. 



The porphyritic augites in these rocks present a character of very 

 considerable interest, which it is necessary to notice here, though I 

 have discussed it in detail in another place (Min. Mag. vol. ix.). 



The augite-crystals exhibit lamellar twinning and subsequent 

 schillerization parallel to the basal plane (001). Similar varieties 

 have been described by William Phillips, Osann, and by Mr. Teall. 

 The forms found in the Western Isles of Scotland differ from those 

 described by the first and last-mentioned authors in not exhibiting 

 simple twinning parallel to the orthopinacoid (001), but in showing 

 some traces of lamellar twinning and subsequent schillerization 

 parallel to that plane. (See Plate XIV. figs. 1 & 2.) 



The occurrence in certain andesites of augite crystals exhibiting 

 lamellar twinning with schillerization has led to a group being estab- 

 lished by some authors bearing the name of " Diallago-andesites." 

 It is x3roi)able that, in many, perhaps in all, of the cases in which 

 diallage is stated to exist in andesitic lavas, augite twinned and 



