OF THE ^VESTEKX ISLES OF SCOTLAND. 



383 



Fig. 1 shows a twinned group of Augite-crystals, exhibiting partings, produced 

 by schillerization, along planes parallel both to the orthopinacoid 

 and the basal plane. The crjstals occur in a type of rock to which 

 the name of " Diallage-Andesite " has been given by some authors. 

 It is from Mingary Castle, Arduamurchan. The specimen is shown 

 as seen with a magnifying-power of 100 diameters. (See p. 363 and 

 ' Mineralogical Magazine,' vol. ix.) 



Fig. 2. Transverse section of a prism of the same Augite, showing the cleavage 

 and the secondary twinning parallel to the orthopinacoid, the planes 

 of the latter being crowded towards the centre of the crystal. 

 Showing, as magnified. 250 diameters. 



Figs. 3, 4, 5, and 6, represent a few of the most striking types of the Older 

 Tertiary Audesite Lavas, as seen in parts of the rock-masses that 

 have undergone a minimum amount of chemical alteration. (See 

 p. 356.) 



Fig. 3. Vitrophyric Augite-andesite, showing groups of crystals of plagioclase, 

 augite, and magnetite (with some apatite), sparsely scattered through 

 a glassy base, which is crowded with beautiful trichites. The latter 

 are in many cases resolvable into globulites. The rock is from 

 Mhaim Clackaig, in Mull, and is shown as viewed with a magnifying- 

 power of 25 diameters. (See p. 362.) 



Fig. 4. Spherulitic Augite-andesite, from Beinn-a-Grhraag, Mull. Crystals of 

 plagioclase, augite (much altered), and magnetite are scattered 

 through a glassy base, showing incipient spherulitcs. These spheru- 

 lites are seen to affect a parallel arrangement, due to the movement 

 of the mass. Magnified 25 diameters. (See p. 362.) 



Fig. 5. Glassy Andesite, from Bealach a' Mhaim, Skye. The black glass is 

 almost as opaque, in thin sections, as that of the basalts (Tachy- 

 lyte). There are many spherulites, consisting each of two concentric 

 zones ; and also spherulitic fringes around the much-corroded 

 plagioclase crystals. This glass is associated with a " labradorite- 

 andesite." Magnified 25 diameters. (See p. 3()4,) 



Fig. 6. Banded Augite-andesite, from Beinu-a-Ghraag, Mull. The fluidal 

 structure in the base of this rock is vei-y beautifully exhibited, and 

 is rendered conspicuous by the manner in which the bands of 

 microlites are seen to curve around the porphyritic crystals. 

 Magnified 25 diameters. (See p. 362.) 



In figs. 7 and 8 an attempt has been made to show the character- 

 istic differences in the effects of solfataric and contact alteration. 



Fig. 7 is the Hornblende-propylite of Beiini Talaidh, in Mull. Scarcely a trace 

 of the original glassy base and plagioclase crystals can now be seen 

 in it ; the colourless ground-mass consisting of secondary felspars 

 and epidotes, in which only occasionally the outlines of the pseudo- 

 morphs of original constituents of the rock can be detected. The 

 hornblende has been converted into mixtures of chlorite and 

 magnetite ; but in these i)scudomorphs traces of the " resorption- 

 halos " originally fornu.'d of pyroxene and magnetite, and consti- 

 tuting sheaths around the ci-ystals, can still be detected. The 

 section is shown as magnified 100 diameters. The peculiarities of 

 the alteration of this rock are midoubtedly due to solfataric action. 

 (See p. 361).) 



Fig. 8 is the Augite-andesite from a point near its junction with the intrusive 

 granite ("granophyre ") of Beinn Uaig, in Mull. Scarcely a trace 

 of the original crystals of the rock can be detected. In a colourless 

 and structureless base, which is anisotropic but not individualized, 

 we find numerous minute roimded granules of a colourless mineral, 

 usually taken for Augite, with many grains of magnetite. A little, 

 strongly plcochroic, brown biotite makes its appearance, and in- 

 creases rapidly in quantity as we approacli the intrusive rock. 

 Magnified 100 diameters. (Seep. 370,) 



