74 PEOr. J. W. JTJDD ON TEETIAEY 



and relations of the rock-masses which exhibit these very dissimilar 

 types of structure which may account for their differences. It has 

 been suggested that the ophitic structure is characteristic of, and 

 limited to, rock-masses which are intrusive in other rocks. While, 

 however, it is certain that the most perfect illustrations of ophitic 

 structure on the grandest scale are found in such splendid examples 

 of intrusive sheets as those of the Shiant Isles and Portrush, 

 yet it is an undoubted fact that many of the larger and more 

 massive of the lava-currents of the district exhibit this same struc- 

 ture in a fairly well-marked manner — every gradation, indeed, 

 existiug between these ophitic and semi-ophitic lavas and the more 

 common type, which is, as we shall see, distinctly granulitic or 

 porphyro-granulitic. Eut in some of the smaller masses, both lavas 

 and dykes, we find this beautiful structure admirably displayed 

 though on a very minute scale. These constitute the micro-o^hitic 

 basalts (see Plate Y. figs. 3, 5, 7). 



The great mass of the doleritic and basaltic lavas, however, ex- 

 hibit different stages of the granulitic and the porphyro-granulitic 

 types. From the granulitic dolerites, which we have described, 

 through the micro-granulitic forms, we get every gradation — the 

 granular elements, pyroxene, and olivine becoming smaller and 

 smaller, as do the lath-shaped felspars, till all are reduced to the 

 condition of microlites ; at the same time the quantity of glassy 

 matter increases, till we pass from the ordinary basalts to the 

 magma-basalts (see Plate V. figs. 2, 4, 6, 8). 



If we now turn our attention from the more crystalline to the 

 compact and vitreous varieties of the basalts (magma-basalts), we 

 find the two parallel series of differences to which we have already 

 referred. It is in the glass of basalt forming dyhes that we find 

 the best examples of the skeleton-crystals of magnetite with those 

 of augite and felspar (see Plate YI. figs. 1, 3, 5, 7) ; while in the 

 basalt of lava-streams the glass is filled with rounded microlites 

 of augite, detached grains of magnetite (magnetite dust), and 

 short prismatic microliths of felspar (see Plate YI. figs. 2, 4, 6, 8). 

 Between these two types there are, of course, all intermediate 

 stages ; but I have no hesitation in afiirming, as the result of pro- 

 longed and repeated studies of these rocks in the field, that the 

 basalt containing a glass with skeleton-crystals is especially cha- 

 racteristic of the dykes, while varieties in which the microliths are 

 short in form and often rounded in outline, constitute the type 

 which is usually exemplified in the lava-streams. 



The tachylytes of the district are only known to occur as " sel- 

 vages " to dykes. In these tachylytes, as has been already shown, 

 skeleton-crystals abound. There is one interesting type, that of 

 the Beal, near Portree, iji Skye, where a magma-basalt, full of 

 skeleton-crystals, passes at the edge of the dyke into a glass in 

 which the magnetite is separated only in the form of cloudy patches, 

 sufficiently dense to render the mass almost perfectly opaque, 



Porphyritic varieties occu]' both among the rocks forming intrusive 

 masses and in those constituting lava-streams. While they are com- 



