OLDER PERILOTITES OF SCOTLAND. 361 



which enclose well-formed crystals of felspar. The crystalline 

 continuity of these large patches of augite is shown by the broad 

 cleavage-fractures, which cause the rock to present a number of 

 brilliant faces reflecting light. Under the microscope the same fact 

 becomes still more obvious by the persistence of the cleavage-cracks, 

 and by the same optical relations being exhibited over considerable 

 areas, when the sections are viewed by polarized light. 



Nowhere is this beautiful ophitic structure more admirably dis- 

 played than in the Shiant Isles, but in a less perfect condition it is 

 seen in the later intrusive masses of Sarsta Beinn in Mull*, and 

 many other localities. "While, on the one hand, the gabbros not 

 unfrequently exhibit some indications of an approximation to this 

 structure, it is also displayed in a far less perfect condition (semi- 

 ophitic structure) in some of the basalts. The careful examination 

 of these rocks with the microscope fully bears out my original 

 conclusion, derived from studies in the field, that the most perfect 

 transitions occur from gabbros through dolerites and basalts to basalt- 

 glass. 



The other structure presented by the dolerites I have ventured 

 to designate as the porphyro-granulitic structure. The base of 

 these dolerites is composed of a mixture of lath-shaped felspars, 

 and small irregular grains of augite, and through this base large 

 porphyritic crystals of felspar and large grains of olivine are dis- 

 tributed in greater or less abundance. A very excellent example 

 of this structure is exhibited by the rock at Eu Geur in Skye t. 



Some of the granitic gabbros exhibit a distinct approach to the 

 ophitic structure in the arrangement of their crystals, while the 

 granulitic gabbros graduate insensibly into the porphyro-granulitic 

 dolerites. Both the ophitic and the granulitic dolerites are found in 

 turn passing insensibly into basalts by the appearance of a more or 

 less glassy base between the crystals. 



I propose to discuss the origin of these structures and the manner 

 in which they shade insensibly into one another, and also into the 

 structures characteristic of the basalts, in a subsequent paper devoted 

 to the description of the whole of the basic rocks of the Western 

 Isles of Scotland. It was necessary, however, to refer to the subject 

 here, as the peridotites in every case exhibit a structure analogous 

 to that of the gabbro or dolerite with which they are associated. 

 Thus a gabbro, by the disappearance of its felspar, is seen passing 

 into a peridotite of granitic structure ; and an ophitic dolerite, in the 

 same way, graduates into a peridotite with ophitic structure. In 

 PI. XIII. fig. 2 exhibits an example of the porphyro-granuHtic struc- 

 ture, figs. 5 & 6 of the granulitic, figs. 4 & 8 of the ophitic, and 

 fig. 7 of the granitic structures, as exhibited in rocks of the ultra- 

 basic class. 



* Quart. Journ. Geol. Soc. vol. xxx. (1875) p. 264-266. 



t Ibid. vol. xxxiii. (1878) p. 692. This dolerite, in its platy structure, so 

 closely resembles a phonolite, that from my examination in the field i was erru- 

 neously led to class it with those rocks. 



