414 PEOF. J. W. JUDB 017 THE TERTIAET AND 



Fig. 9. Section of a crystal of typical hypersthene from Labrador, seen witli a 

 magnifying-power of 30 diameters. Tiiree sets of tabular enclosures, 

 which are of extraordinary dimensions, are seen in this section, two 

 in section and one in plan. One set of enclosures, seen in section, 

 is very persistent over the whole area ; a second, nearly at right 

 angles, is developed along certain bands, probably of strain ; and the 

 third, seen in plan, are also crowded along the same lines. The en- 

 closures seen in plan have often one straight edge (due to the limits 

 of the negative crystals in which they are developed) ; but on the 

 other sides they are irregular in outline, owing to the secondary 

 materials of which they are formed not entirely filling the negative 

 crystal. The material of the crystal in which these enclosures lie 

 has lost nearly every trace both of colour and of pleochroism. (See 

 page 380.) 



Plate XII. 

 Fig. 1. Eepresents the surface of a crack traversing a crystal of olivine in the 

 gabbro of Halival, Isle of Rum, as seen with a magnifying-power of 

 225 diameters. The irregular cavities sometimes contain a liquid 

 and bubble, and at other times are empty ; more usually, however, 

 they are filled, to a greater or less extent, with dark-coloured, solid 

 materials which appear to be decomposition-products. By the 

 use of very high powers and special means of illumination fine 

 reticulating tubules can be detected uniting many of these cavities. 

 (See page 382.) 



2. Shows the surface of a crack traversing an olivine crystal in the picrite 

 of central Eum. In addition to cavities filled with decomposition- 

 products, we find a beautiful dendritic network of magnetite and 

 other oxides, spreading itself over the surface of the crack. The 

 object is shown as it appears with a magnifying-power of 100 dia- 

 meters. (See page 382.) 



3.. Portion of crystal of olivine from the picrite of central Eum, magnified 

 50 diameters. Showing enclosures along cracks filled with solid 

 substances, as in fig. 1 ; dendritic ramifications of magnetite over 

 the planes of cracks, as in fig. 2 ; and, in addition, numerous en- 

 closures in negative crystals arranged in a series of parallel planes 

 traversing the crystal. (See page 382.) 



4. Very large and beautiful examples of the stellate bodies lying within the 



negative crystals in olivine, from the picrite of Halival, Isle of Rum. 

 Magnified 75 diameters. In the crystals on the left, the stellate en- 

 closures are viewed in plan and their forms are well seen ; in the 

 crystal on the right they are gi-eatly foreshortened. When the 

 plane of the section is at right angles to that of the plane parallel to 

 which the negative crystals lie, the enclosures appear as fine dark 

 lines. This section also shows the lines of cavities filled with solid 

 substances passing along cracks in the olivine-crystals. (See page 

 382.) 



5. A series of examples of very fine and large stellate enclosures in olivine 



from the picrite of Halival, Isle of Rum. Magnified 225 diameters. 



(See page 385.) 

 5 a. Illustrates the first stage in the formation of the stellate enclosures in 



the negative crystals in olivine. The ramifying rods of magnetite 



&c. are assuming a radiate arrangement, but have not united with one 



another. 

 5b. Shows a very large and perfect stellate enclosure ; the edges have not 



extended to the limits of the negative crystal, and therefore are 



not bounded by regular lines. 

 5c. Shows two stellate enclosures, the growth of which has been interfered 



with by the sides of the negative crystal. They also illustrate the 



tendency of the stellate enclosures to pass into tabular ones by 



additional deposits between the rays of the star. 

 5d. Illustrates the efiects of foreshortening on the apparent forms of these 



stellate enclosures. 



