East of Gelli Hill, Radnorshire. 505 



dolerite is a fact of some considerable importance in the geology of 

 Wales and Shropshire. 



The Glan-oer Type. 



At Glan-oer is exposed a fine-grained igneous rock, dominantly 

 pale grey, but with small black specks and larger whitish spots. 

 The same type is exposed in the quarry between Little ISTant and 

 Graig-fawr, also in a dyke running N.W.-S.E. from the ford in the 

 Nant Brook below Hendy Bank to a point south-west of Llanwefr 

 Pool. The specific gravity is 2-69. 



In thin section the rock is seen to be made up largely of a felted 

 mass of felspar laths with somewhat ragged outlines. They are very 

 constant in length, averaging about 0'7 mm. Alteration has gone on 

 to a considerable extent, but they can be seen to polarize in first order 

 colours and to exhibit lamellar twinning. Symmetrical extinction 

 angles are always low, the maximum value observed being 12°; the 

 felspar must approximate to oligoclase in composition. Another 

 conspicuous constituent is a pale-green alteration product which is 

 sometimes moulded on the felspars ; the relief is not great, but the 

 refractive index is distinctly higher tlian that of Canada balsam ; 

 polarization is first order grey and is of the aggregate character. 

 There is an occasional suggestion of olivine in the shape of the 

 pseudomorphs, and one particular case (l-2mm. in length) places the 

 matter beyond doubt; the substance is serpentine, and is the result 

 of the alteration of olivine, almost certainly a variety poor in iron. 

 It is possible that some of the felspar crystallized before the olivine, 

 though the moulding of the serpentine on the felspar may be due to 

 tlie expansion consequent on the alteration. In this connexion, 

 though, it must be borne in mind that Professor Watts,' in dealing 

 with the olivine-dolerite dykes of Antrim, has described a case 

 where the felspar crystallized before the olivine. Some varieties of 

 the allivalite of Dr. Harker* also exhibit a similar sequence of 

 crystallization. In that rock, however, the felspar is anorthite. 



Another interesting constituent is a pale-brown augite. This 

 mineral is totally absent from some of the sections, and even in the 

 case of those in which it occurs the distribution is somewhat 

 eccentric. It is found in spots only large enough to enclose, perhaps, 

 a score of felspar laths. When altered, it produces a cloudy 

 aggregate containing much calcite ; this is responsible for the whitish 

 spots visible to the naked eye. Other constituents are apatite^ 

 fairly plentiful secondary sphene, and pyrite. 



The remaining mineral is clear and fresh, with a refractive index 

 approximating to that of Canada balsam ; it polarizes in first order 

 grey with occasional yellow. Careful search resulted in the 

 discoveiy of a definite cleavage, lamellar twinning, and the fact that 

 the refractive index is distinctly below that of Canada balsam. The 

 mineral is biaxial and the birefringence is positive. It is undoubtedly 

 albite. Twin lamellae run interruptedly from one felspar to the 

 other, though the angle of extinction changes. It is not possible at 



' Guide to Rocks and Fossils (Geo). Snrv. Ireland, 1895, p. 78). 

 2 Petrology for Students, 4th ed., 1908, p. 103. 



