146 



course cannot be detected by the method under consideration. The general 

 conclusion, therefore, at which we arrive, is that the determination of the 

 precise character of the minute felspars in rocks is in most cases impracticable. 

 All that we can say is that those which give lath- shaped sections and very 

 low extinction angles are allied to oligoclase, and those which give similar 

 sections and high extinction angles are allied to labradorite and anorthite. 



We have now to consider the distribution of the different varieties of 

 plagioclase in the different classes of basic igneous rocks. As we should 

 naturally expect, the original felspars of the basic rocks are of a basic 

 character. Anorthite has been analysed by Dr. HAUGHTON from a 

 plagioclase-augite rock (eucrite) from Carlingford, County Down ; and Dr. 

 HEDDLE gives an analysis of an altered anorthite from the gabbro of 

 Lendalfoot, in Ayrshire. Typical anorthite appears, however, to be 

 comparatively rare, the most common felspar being one intermediate in 

 composition between anorthite and labradorite. TSCHERMAK and SCHUSTER 

 use the term bytownite for such a felspar. In the coarse-grained granular 

 rocks, belonging to the gabbro- family, the plagioclase occurs in the form of 

 grains of tolerably uniform dimensions in the different directions, or as large 

 crystalline plates, containing inclusions of other minerals, especially olivine. 

 It shows broad lamellar twinning on the albite, and less frequently on the 

 pericline plan. 



In porphyritic rocks belonging to the dolerite-family it occurs as 

 individual crystals and as crystalline granular aggregates, which are sometimes 

 completed externally, as in the Tynemouth dyke <l) by felspar-substance of a 

 composition somewhat different from that forming the interior portions of 

 the grains. The following list of analyses will give an idea of the composition 

 of the felspars of the basic eruptive rocks of Great Britain and Ireland. 



MnO ... ... ... ... 0-913 



99-25 99-86 100-486 100-312 99-847 100-97 



I. Anorthite from Eucrite of Carlingford. Dr. HAUGHTON. Phil. Mag. 



Series IV. Vol. XIX., 1860, p. 13. 



II. Bytownite occurring as porphyritic granular aggregates in the Tynemouth 



Dyke. Mr. STEAD. Q.J.G.S., Vol. XL., 1884, p. 234. 



III. Bytownite from Gabbro, Hart O'Corry, Skye. Dr. HEDDLE. Trans. 



Eoy. Soc. Edin. Vol. XXVIIL, 1879, p. 253. 



(1) See TEALL, Petrological Notes on some North of England Dykes. Q.J G.S , Vol. XL. 

 1884. p. 233. 



