173 



predominates. Microscopic examination shows that the felspar is tolerably 

 fresh, though frequently traversed by cracks, and that the serpentine is 

 altered oiivine containing kernels of the undecomposed substance. A small 

 amount of diallage is present. In one slide the olivines were seen to be 

 surrounded by a fringe-like border, consisting of two zones, an inner and an 

 outer. The outer zone was composed of needles of actinolite or tremolite. 

 Here we have a feature similar to that first described by TORNEHOHM. (i) 

 in the " hyperite " of OLME, and subsequently noticed by J. BKCKE in the 

 olivine-gabbros of Langenlois, (2) in lower Austria, and ]iosswein (3) in 

 Saxony ; also by G. H. WILLIAMS, in the peridotites near Peekskill, (4) and 

 the gabbros (pyroxene-granulites) of Baltimore. (5) In the last mentioned 

 case, however, the fringes occur round hypersthene instead of oliviue. In all 

 cases they are limited to the zone of contact of oiivine (or hypersthene) 

 and felspar ; so that no doubt can arise as to their being due to a reaction 

 between the constituents of these two minerals. 



Gabbros occur also in the Carrick district of Ayrshire. (6) They consist 

 of diallage and saussurite, and are exposed on the coast north of Lendalfoot. 

 The saussurite is a dense white compact substance which is, as a rule, 

 somewhat opaque under the microscope, and not capable of being resolved 

 into distinct minerals. Here and there some colourless felspar containing 

 acicular microlites may be detected. The diallage is partly replaced by 

 uralite and actinolite. Associated with the gabbro is a remarkable rock, 

 composed almost entirely of large sub-metallic crystals of diallage, frequently 

 measuring two or three inches in length. Gabbro occurs also at Portsoy, in 

 Banffshire, in the Islands of Unst and Fetlar, belonging to the Shetland 

 group. I7) It is doubtless present in may other Scotch localities. Professor 

 ROSEN BI'SCH mentions the occurrence of exceptionally beautiful norites at 

 Kirkhill and Towie Wood, north-west of Ellen, in Aberdeenshire. 



A group of rocks composed of two or more of the minerals plagioclase, 

 diallage, a strongly pleochroic rhombic pyroxene, biotite, magnetite or 

 ilmenite and hornblende, enters very largely into the composition of the 

 Hebridean gneissic system of Sutherlandshire, in the neighbourhood of Scourie. 



(1) N.J., 1877, p. 383. 



(2) T.M.M., Neue Folge. Vol. IV., 1882, p. 355. 



(3) Ib'nl, p. 450. The Rossweiu case is particularly interesting. The fringe is 

 sometimes five or six millimeters wide. It is radially fibrous in structure, and consists of two zones. 

 The inner zone shows a silky lustre. Detached portions give straight extinction, a symmetrical 

 interference figure, and a negative bisectrix. In thin sections this /one is seen to consist 

 of colourless individuals, possessing the cleavage of hornblende, and giving in crocs section an 

 interference figure indicating widely separated optic axes. These characters show that 

 the mineral is a rhombic hornblende (anthophyllite). Magnetite occurs abundantly between the 

 fibres of anthophyllite. The exterior zone is composed of green fibres which possess the 

 extinction and other characteristics of actinolite. No magnetite occurs in this zone ; the whole 

 of the iron having been absorbed in the actinolite. 



(4) A.J.S. Vol. XXXI., 1886, p. 35. 



(5) Bull, U.S. Geol. Survey. No. 28 (1886), p. 43. 



(6) BONNET. Q.J.G.S., Vol. XXXIV., 1878, p. 769. 



(7) See Dr. HEDDLE. Trans. Roy. Soc., Edin. Vol. XXVIII. 



