412 PEOF. J. TT. J TDD 0^- THI TEETIAET A>-D 



in the unt\Tiniied portioBs, like the similar twin-structures artificially 

 produced in calcite, is also seen. Some of these twin-structures may 

 have been developed by the strains set up in the cooling-down of the 

 crystal, others were probably induced by the movements in the rock- 

 mass which produced the fissures in the crystals that built it up. 

 (See pages 36-J^366.) 

 Fig. 2. Section of felspar in troctolite from Hahval, Isle of Rum. Magnified 

 225 diameters. This ciystal exhibits the first stages of alteration in 

 the felspars. It is traversed by numerous cracks,, and along these, as 

 well as in other parts of the crystal, many liquid- and gas-cavities 

 have made their appearance. Some of these cavities, which are of 

 considerable size, are particularly interesting from the fact that they 

 are seen by their regular forms to be negoMve crystals. In some 

 cases the infilling of these cavities with solid substances has clearly 

 commenced. (See pages 375, 376.) 



3. Section of felspar from the olivine-gabbro of Loch Coruiskh, Isle of Skye. 



Magnified 225 diameters. This felspar is in a more advanced state of 

 alteration. The cavities lying along lines of fissure or strain are 

 much more numerous and are in almost all cases filled with dark- 

 coloured products of decomposition. In addition, we see the first 

 traces of very minute dark-coloured tabular enclosiu'es (negative 

 crystals filled with decomposition -products) making their appearance 

 along one set of parallel planes within the crystal. (See page 376.) 



4. Section of felspar from the oHvine-gabbro of Ardnamm-chan. Magni- 



fied 225 diameters. This section shows bands of cavities, some of 

 which are partially filled with sohd materials, and tico series of 

 tabular enclosm-es (infilled negative crystals) arranged in two sets of 

 parallel planes which intersect one another nearlj- at right angles. 

 (See page 376.) 

 o. Another section of felspar from the same rock as the last, showing the 

 enclosures arranged in two sets of planes, as in the former example, 

 with the addition of a third series intersecting them. This section is 

 also magnified 225 diameters. (See page 376.) 



6. Section of felspar from the olivine-gabbro of Loch Coruiskh. Magni- 



fied 225 diameters. This shows irregular enclosures filling cavities 

 which lie along cracks and scattered irregularly through the crystal ; 

 also tabular enclosures arranged along no less than five different 

 intersecting planes in the crystal, namely, two pinacoidal, two pris- 

 matic, and one basal. The fii'st four are seen in section, the last in 

 plan. Besides these there are cloudy patches in the crystal, which the 

 highest powers employed are only partially capable of resolving 

 into similar but much more minute enclosures. (See page 376.) 



7. Section of an excessively altered crystal of felspar from the olivine- 



gabbro of Ardnamurchan. Shown with a magnifying-power of 225 

 diameters. The whole of the felspar exhibits a rich brown tint from 

 the abimdance of foreign enclosm-es which have been deyeloped in it. 

 These enclosures form nebulous-brown patches, which, with the verj- 

 highest powers of the microscope, can only be partially resolved into 

 irregular solid particles h-ingin cavities of the crystal, and tabular or 

 bacillar enclosures, filling negative crystals and developed along a 

 number of planes within the felspar. In other portions of the 

 section, the enclosm-es are sufficiently large to be rendered visible by 

 the power employed. A further concentration of the decomposition- 

 products has taken place in places, leading to the formation of dendriti- 

 form accumulations of the iron-oxides. &c. Although the crystal is so 

 gi-eatly altered, the characteristic lamellar twinning can be observed in 

 certain portions of it with the aid of the polariscope. This lamellar 

 twinning has evidently influenced the action of the solvent forces in 

 eating out negative crystals along certain planes, and in iafilling them 

 with decomposition-products. This example affords an ea.sy transi- 

 tion to the chatoyant felspars, in which the secondary structures are 

 e//?'ra-microscopica]. (See pages 376, 377.) 



