OF THE WESTERN ISLES OF SCOTLAND. 359 



basalts. It is among these lava-sheets, at a considerable distance 

 from the great eruptive centres, that these rocks can be studied in 

 their least altered condition. Around the great igneous centres the 

 extreme of alteration is seen to have taken place, not one of the 

 original minerals of the rocks being recognizable except as pseudo- 

 morphs. The felspars are usually completely kaolinized, the ferro- 

 magnesian minerals represented by chlorites, while even the titano- 

 ferrite is converted into white opaque products, and finally into the 

 transparent leucoxene or titanomorphite (sphene?). 



Eosenbusch has proposed to divide the amphibole-andesites into 

 two groups — those which in addition to the amphibole or mica, also 

 contain a pyroxene, and those in which pyroxenes are absent. Both 

 of these groups appear to be well represented in the Western Isles 

 of Scotland, llosenbusch has also proposed to divide the amphibole- 

 and mica-andesites containing pyroxene into two groups — those in 

 which the pyroxene is an augite or monoclinic variety, and those in 

 which it is an enstatite in rhombic form. The latter type, which 

 is so abundantly represented in the recent volcanic rocks of the 

 Western Territories of the United States, and southward in the 

 Bepublic of Salvador, according to Hague and Iddings *, is beauti- 

 fully illustrated in the district which we are describing. 



In the HonibUnde-andesites proper we find a " microlitic felt " of 

 felspar needles, through which are scattered groups of green horn- 

 blende crystals, often assuming sheaf-like and tufted groupings. 

 In most cases it is clear that each hornblende crystal or group of 

 crystals was originally surrounded by a resorption-halo, that is, a 

 sheath composed of pyroxene and magnetite, the result of the action 

 of heated magma on the hornblende. But in most cases the 

 pyroxene has been converted into isotropic viridite or into a chlorite 

 of feeble double refraction, while the granules of magnetite still 

 surround the more or less altered hornblende. In many instances, 

 the hornblende can be seen to have been completely changed into a 

 chlorite, with the separation all through its substance of granules of 

 magnetite. These chlorite-pseudomorphs after hornblende, with 

 granules of magnetite crowded along their sides and also scattered 

 through their midst, arc very characteristic of the propylites which 

 are derived from the hornblende-andesites (see Plate XIV. fig. 7). 



One of the best types of this group is found in the rock of 

 Beinn Talaidh (Beinn Talla) in Mull. This rock varies in specific 

 gravity from "2-iSO in the least crystalline types to 2*08 in those 

 more highly crystalline. In the deeper corries of the mountain, 

 andesites arc found exliibiting a distinctly dioritic habit. The 

 chemical composition of this rock is shown by the analysis given 

 at p. 349. 



The hornblende-andesites of the Western Isles of Scotland 

 exhibit the widest diversity in the proportions of their constituent 

 minerals. Some of the rocks of this class, good examples of which 

 may be seen near Salen, Mull, consist very largely of felspar crystals 



* Amer. Journ. Sci. vol. xxvi. (1883) p. 233, vol. xxvii. (1884) p. 400, 

 vol. xxxii. (1886) p. 28. 



2d2 



