184 



tions of this type occur close to the landing places at Portree and Tobermory, 

 in the Shiant Isles, near Ardtornish Castle, and at Portrush in Antrim. 



Rocks of precisely the same character have been described by BREON as oc- 

 curring abundantly in Iceland^and the Faroe Isles. This author isolated the 

 lath-shaped felspars and determined them by chemical analysis to be labrador- 

 ite. In thin sections they frequently give extinctions of about 30. There is 

 therefore no doubt that this special type of dolerite is highly characteristic 

 of the Tertiary volcanic district including Iceland, the Faroe Isles, the west 

 of Scotland and the north of Ireland. If Figures 1 and 2, Plate X., of the 

 present work be compared with Figures 1 and 2, Plate I., of M. BREON'S work 

 on Iceland, the striking resemblance of the Scotch to the Icelandic rocks will 

 be at once apparent. In addition to the ophitic dolerites we find others of 

 the type with granular augites. The relation of these to the ophitic dolerites 

 is sometimes clearly shown. Differential movement in an ophitic dolerites 

 when it is near the point of final consolidation must lead to the breaking up of 

 the irregular masses of augite into a number of grains and granules having in- 

 dependent optical characters. That such is the origin of some of the dolerites 

 and basalts with granular augites is proved by the fact that transitions from 

 the one type of structure to the other may be obtained in the same rock-mass 

 and sometimes also in the same slide (see Fig. 2, Plate X.). Professor JuDD< 2 > 

 has called attention to the existence, in the area now under consideration, 

 of dolerites which approximate in structure to the trap-granulites of Saxony. 

 " The most notable distinction in these rocks," he says, " is found in the 

 character of the pyroxene and olivine grains, which, as seen in section, 

 assume more or less rounded outlines, and are embedded in a plexus of 

 lath-shaped crystals of felspar ; in polarised light these grains are seen not 

 to be parts of one large crystal, but to have very different orientations." 

 Prof. JUDD proposes that the dolerites in question should be said to be gran- 

 ulitic. The ophitic structure is comparatively rare in the basalts, the augite 

 for the most part taking the form of more or less rounded grains. 



By a diminution in the sizes of the different constituents the dolerites 

 pass by imperceptible gradations into the fine-grained and compact basalts. 

 These present us with the same types of structure as the dolerites. Intersti- 

 tial matter may be present in rocks presenting any of the above textural 

 characteristics. When present in considerable quantities the term magma- 

 basalt may be conveniently used. The gradual passage of magma-basalt 

 into basalt-glass has been described by Professor JUDD, and the cha- 

 racters of this glass have been investigated by him in conjunction with Mr. 

 CoLE. (3) In no case is basalt-glass known to occur on an extensive scale in 

 the West of Scotland. It forms narrow selvages, at most only 2 or 3 inches 

 thick, to certain basaltic dykes. At present it appears to be recorded only 

 from seven localities ; at the Beal near Portree, in the Isle of Skye ; at Lam- 

 lash (Holy Isle) Arran ; at Some Point on the N.W. coast of the Isle of 

 Mull ; at Gribun in the west of Mull ; at Screpidale in the Isle of Raasay ; 



(1) Geoloffie de PIsland, Paris, 1884. 

 2) See JUDD, Q.J.G.S., Vol. XLII. (1886), p. 49. 



(3) On the Basalt-glass (Tachylyte) of the Western Isles of Scotland. Q.J.G.S. Vol 

 XXXIX., 1883, p. 444. 



