OLDER PEPJDOTITFS OF SCOTLAND. 



413 



Plate XI. 



Fig. 1. Fissures and incipient fissures in the very fresh brown augite of the 

 ophitic dolerite from the Shiant Isles, showing the development of a 

 reticulation of cavities along the lines of fracture and strain within the 

 crystal. Most of these cavities are empty, or contain liquids in which 

 bubbles may sometimes be discerned ; but a few of them appear to be 

 filled with solid substances. These cavities are very minute ; they are 

 shown as seen with a magnifying-power of 500 diameters. (See 

 page 378.) 



2. More altered brown augite from the picrite of central Rum. The 



cracks and bands indicating strain are more numerous than in the 

 last example, and are marked by lines of cavities of much larger 

 dimensions, the cavities being in almost all cases filled with solid 

 decomposition-products which are dark-coloured and opaque. The 

 section is shown as displayed by a magnifying-power of 75 diameters. 

 (See page 378.) 



3. Two crystals of augite from the olivine-gabbro of Loch Coruiskh, Isle 



of Skye, showing the conversion, to a different extent in the two 

 cases, of this mineral into diallage. The action by which enclosures 

 are developed along planes parallel to the orthopinacoid is clearly 

 seen to be set up from the outer surface of the crystal. The central 

 parts of the crystal have all the chai-acters of ordinary aiigite, while 

 the peripheral portions are converted into true diallage. The objects 

 are figured as seen with a magnifying-power of 50 diameters. (See 

 page 379.) 



4. Crystal of true diallage (foliated augite) from the olivine-gabbro of 



Beinn More, Isle of Mull. At one end of the crystal the develop- 

 ment of another series of enclosures along the clino-pinacoid has 

 commenced, conver*"ing the crystal into pseudo-hypersthene. Seen 

 as magnified 100 diameters. (See page 379.) 



5. Portion of crystal of augite from the olivine-gabbro of Loch Coruiskh, 



Isle of Skye. This example shows that the development of enclosures 

 takes place most abundantly along lines of cracks and in their imme- 

 diate vicinity. Two sets of enclosures are in course of development 

 in this case — one parallel to the orthopmacoid, and the other parallel 

 to the clino-pinacoid. Seen with a magnifying-power of 75 diameters. 

 (See page 379.) 



6. Structure of the pseudo-hypersthene from the olivine-gabbro of Loch 



Coruiskh, Isle of Skye. Represented as seen with a magnifying- 

 power of 225 diameters. Two sets of enclosures are seen in section, 

 lying in planes nearly at right angles to one another. A third 

 much less perfect series of enclosures, with irregular outline, is ex- 

 hibited lying probably parallel to the basal plane. (See page 380.) 



7. Crystal of highly ferriferous enstatite (amblystegite) from the olivine- 



gabbro of Loch Coruiskh, Isle of Skye. The crystal exhibits only 

 the first traces of Schillerization. It exhibits the very strong pleo- 

 chroism and the rhombic extinction characteristic of the species to 

 which it belongs. Cavities filled with solid enclosures are developed 

 in great numbers along the lines of crack, and a few tabular en- 

 closures are developed along one set of parallel planes, so that the 

 mineral is seen to be passing into the bronzite-modification. The 

 crystal is shown magnified 30 diameters. (See page 380.) 



8. Crystal of altered ferriferous enstatite, enclosing both felspar and 



diallage, from the olivine-gabbro of Loch Coruiskh, Isle of Skye. 

 The crystal is slightly serpentinized in places. This crystal exhibits 

 the bronzite-modification over the greater part of the section, one 

 very conspicuous series of enclosures, seen in section, being well 

 developed in it ; but here and there a second set of enclosures, also 

 seen in section, and a third, viewed in plan, are also exhibited. The 

 crystal therefore illustrates the transformation of the bronzite- 

 modification of enstatite to the hypersthene-modification. Mag- 

 nified 100 diameters. (See page 380.) 



