68 PEOP. J. W. JUDD ON TEETIAEY 



this structure, the only minerals present (if we exclude those of 

 secondar}^ origin) are felspar, augite, and olivine. The growth of the 

 crystals of these several minerals has taken place in such a way, that 

 their full development has been interfered with by the consolidation 

 of the neighbouring minerals. Nevertheless, the felspar seems to have 

 had a distinct advantage in the order of its development over the 

 other minerals. This is shown by the fact that when any crystal- 

 faces are produced they are those of the felspars. Not only are the 

 augite and olivine moulded on to the felspar-crystals, but in some cases 

 the felspar-crystals may be found to have separated along cleavage- 

 planes, and the augite-substanee has penetrated into, and crystallized 

 within, the spaces thus formed (see Plate YII. fig. 1). The felspar- 

 crystals, in the most perfectly granitic types, appear in section as broad 

 plates, sometimes exhibiting zonal structure, and not in narrow lath- 

 shaped forms. The size of the crystalline grains varies greatly : in 

 Ardnamurchan and Skye, the individual grains sometimes exceed an 

 inch in diameter, but sometimes they are so fine as to be almost 

 microscopic. These finer varieties have been distinguished as ''• micro- 

 granitic''^ or '^ euriticJ' 



2. The Opliitic Structure (see Plate Y. fig. 1), which was first 

 clearly defined by IM. Michel-Levy, is very admirably illustrated in our 

 series of rocks. In the rocks of this type the crystals of felspar, 

 which are lath-shaped in section, were evidently the first formed, for 

 they are often found to be completely enclosed in large individuals of 

 pyroxene and sometimes of olivine and magnetite. The enclosing 

 pyroxene crystals sometimes attain considerable dimensions, but 

 seldom, or never, have their outer faces well developed ; in spite of 

 the felspar-crystals enclosed in them, however, the persistence of 

 cleavage-cracks and the mode of polarization shows the crystalline 

 continuity of the whole individual. "Where these crystalline indi- 

 viduals of pyroxene attain large dimensions, as in the rocks of 

 Portrush and the Shiant Isles, the reflection from their broad 

 cleavage-faces in fractured surfaces causes the rock to assume a 

 striking and characteristic appearance. In Mull, certain lavas are 

 found exhibiting this structure on a very minute scale, and these 

 may be distinguished as micro-ophitic (see Plate Y. fig. 5). 



3. The Granulitic Structure (see Plate Y. fig. 2). — In the 

 rocks exhibiting this type of structure, we sometimes, but by no 

 means always, find magnetite added to the other three original 

 constituents of the rock. The most notable distinction in these 

 rocks is found in the character of the pyroxene and olivine grains, 

 which, as seen in section, assume more or less rounded outlines, and 

 are imbedded in a plexus of lath-shaped crystals of felspar; in 

 polarized light these grains are seen not to be parts of one large 

 crystal, but to have very different orientations. The form of the 

 individuals of pyroxene and olivine at once recalls the structure 

 seen in the granulites. Although never so coarse-grained as some 

 of the granitic forms, the granulitic varieties exhibit a wide diver- 

 gence in the size of the individual minerals, and the finer varieties 

 ma}^ be distinguished as micro-granuUtic (see Plate Y. fig. 6). 



