346 A. B. Hunt — Devonian Hocks of South Devon. 



Appendix. 

 Notes by Alfred Barker, Esq., M.A., 



Fellow of St. John's College, Cambridge. 

 (26) Ai. Sandquay Quarry. 



This is a much altered diabase, the original augite and felspar being almost 

 totally destroyed. Here and there traces of a characteristic ophitic structure can be 

 discerned ; but there has also been a porphyritic development of larger crystals of 

 felspar, and to a less extent of augite, as indeed is seen in the hand-specimen. 

 Eare needles of apatite and grains of irou-ore are to be reckoned among the original 

 constituents. 



The most interesting feature is seen in the numerous little veins which traverse 

 the slide, clearly representing cracks tilled by secondary minerals. The clear material 

 of these little veins seems to be in some places quartz, but most of it is felspar with 

 evident twin-lamellation. This mineral has generally grown perpendicular to the 

 walls of each vein, but wheie the cracks traverse porphyritic crystals of felspar, the 

 new felspar substance is oriented in the same way as the old, with its twin-lamelbe 

 parallel to the almost obliterated twinning of the original crystals. A considerable 

 amount of pale fibrous amphibole is enclosed by the felspar and quartz of the veins, 

 the fibres set perpendicularly to the walls of the vein in each case. Another mineral 

 enclosed is a chloritic substance in vermicular growths. 



The appearances in this rock do not demand any agency beyond those involved in 

 the ordinary processes of ' weathering. ' The secondary growth of felspar is now 

 well known in rocks in which there is no reason to suspect any ' metamorphism ' as 

 ordinarily understood. 



(31) Aj. Combe Cross Quarry, Higher Fug e, West of StoTce Fleming. 



This rock is a diabase less decomposed than that of Sandquay. A considerable 

 amount of fresh, light-brown augite remains, and is seen to be penetrated by the 

 lath-shaped sections of decomposed felspar in the ophitic fashion so characteristic of 

 diabasic rocks. There is no purphyritic structure in this specimen. Iron-ores are 

 rather abundant, and in reflected light the black ground is seen to be crossed by white 

 or grey bars in the manner which has been supposed to indicate a parallel intergrowth 

 of magnetite and ilmenite. As before, there are veinlets consisting of clear lamellated' 

 secondary felspar, and a rather fibrous amphibole. The latter is here better developed 

 than in the preceding slide, and has a pale-greenish to brownish tint. Both minerals 

 have grown perpendicularly to the walls of the little cracks. The pale secondary 

 amphibole is also seen in places forming a narrow fringe to the augite plates, and 

 showing the usual crystallograpbic orientation with respect to that mineral. This is 

 a phenomenon met with in some districts which have been subjected to more or less 

 mechanical stress, ' but it cannot be urged as a proof of dynamic metamorphism. 



(33) Aj. Mill Bill Copse Quarry, Blackpool Valley. 



This is a schistose rock, precisely such as would result from the crushing of the 

 diabase Aj. The schistosity, well seen in the hand-specimen, is marked in the slice 

 by a general parallel arrangement of the constituents, and especially of those of 

 secondary origin. The most abundant of these are a greenish-yellow mineral of the 

 chlorite family and a pale amphibole. The latter has no longer the fibrous structure, 

 but is compact, with evident hornblende-cleavage. Its absorption colours are pale 

 bluish-green parallel to the 7-axis, and pale-brown perpendicular to that direction. 

 The original ophitic structure of the diabase is preserved in certain little patches 

 like 'eyes,' in which light-brown augite is seen, penetrated in the usual fashion 

 by elongated crystals of felspar, mostly decomposed. A few felspars are still clear 

 enough to show the twin-lamellation, and there are also relics of the original iron- 

 ores, which are shown by the development of semi-opaque leucoxene to have been 

 partly titaniferous. The augite, and to some extent the iron-ores, are traversed by 

 fissures roughly perpendicular to the schistosity of the rock, a result of ' stretching ' 



' e.g. Eastern Caernarvonshire : cf. Harker ; Bala Volcanic Series of Caernarvon- 

 shire, pp. 83, 117; 1889. 



