U7 



IV. Labradorite from gabbro, Loch Scavaig, Skye. Dr. HEDDLE (loc. cit.) 



V. Labradorite from diorite, Portsoy. Dr. HEDDLE (loc. cit.) 



VI. Labradorite from gabbro, Loch Scavaig. Dr. HAUGHTON. Dublin 



Quart. Jour. Sci., Vol. V., 1865, p. 94. 



We have at present no very precise determination of the felspars of the 

 finer grained non-porphyritic rocks belonging to the dolerite-family. The 

 ophitic dolerites or, in other words, the plagioclase-augite rocks in which the 

 felspar is present in elongated columnar crystals, and the augite in large 

 irregular crystalline masses (1) are very common, both amongst the palaeozoic 

 and tertiary igneous rocks of this country, and there is no reason to doubt that 

 the felspars of these rocks are identical with those of the corresponding rocks 

 on the Continent. The dominant felspar of the ophitic dolerites (diabases) of 

 the Hartz have been shown by SCHILLING (2) to be typical labradorite, com- 

 posed of two molecules of anorthite and one of albite (Ab! An 2 ). 



There can be no doubt, therefore, that the prevailing felspars in the basic 

 division of the normal plagioclase rocks belong to the labradorite-anorthite 

 group. At the same time there is evidence that felspars more nearly allied to 

 albite are present in certain members of the group, especially those which 

 occur on the border line between the rocks of normal basic and those of normal 

 intermediate composition. The Whin Sill may fairly be classed as a member 

 of the basic family, for it possesses a silica per-centage of about 50 and a 

 specific gravity over 2 '9. A consideration of the analysis of the impure felspar 

 substance, extracted from this rock by the specific gravity method, points to the 

 conclusion that the original felspar was andesine. (3 ) This rock, although it 

 occasionally shows traces of the ophitic texture, does not possess this texture 

 in anything like the same degree of perfection as the dolerites in which the 

 felspar is known to be labradorite. Another point of importance in relation 

 to this rock is the presence of the bisilicate, enstatite, in the place of the uni- 

 silicate olivine. Dr. HA WES has shown that the isolated felspars of the rock 

 from Jersey City which possesses many points of resemblance to the Whin Sill, 

 can be separated into two portions, the one having a specific gravity greater 

 than, and the other less than 2'69. The former on analysis proved to be 

 labradorite and the latter andesine. (4) 



The occurrence of oligoclase in the rocks of the basic division is certainly 

 rare. Nevertheless the microlitic felspars of certain trachytic representatives 

 of this division give approximately straight extinction in all longitudinal 

 sections, and may therefore be referred to this species. Albite is not known 

 to occur as an original constituent, but appears to be frequently present, as a 

 secondary product, in those rocks that have been affected by regional metamor- 

 phism. Orthoclase has already been mentioned as a constituent of certain 

 " exfiltration " or " contemporaneous " veins in the olivine-dolerites of 

 Rowley Regis in Staffordshire. Dr. GEIKIE mentions the occurrence of 



(1) Structure diabasich-koring (ROSENBUSCH") and divergent strahlig (LosSEN). 



(2) Grunsteine des Harzes. Gottingen, 1869. 



(3) TEALL. Q.J.G.S., Vol. XL., p. 642. 



(4) Proceedings of IJ. S. Natural History Museum, 1881, p. 129. N.J., Itef., 1882, Band 

 I., p. 414. 



