246 II. Dewey Sf Dr. J. S. Flett— 



minverite, quartz-diabase, spilite, keratophyre, quartz-keratophyre, 

 soda-felsite, and albite granite, ranging from ultra-basic to acid in 

 composition. 



8. Their essential characteristics are the abundance of soda-felspar, 

 and the remarkable frequency with "which they have been albitized. 



4. The albitization is not characteristic of the whole suite, but is 

 especially frequent in certain members of it, such as the spilites and 

 diabases, while others like the quartz-diabases are less liable to this 

 change. It is not due to weathering or shearing. Good evidence 

 exists to prove that the albitization took place soon after the rocks 

 had solidified, and consequently it may be grouped among the post- 

 volcanic or juvenile changes of rock-masses. 



5. The constant association of adinoles (albitized shales) with the 

 albite-diabases, and of radiolarian cherts with pillow-lavas, finds 

 a simple explanation on this hypothesis, and at the same time affords 

 the strongest confirmation of it. 



6. The composition of the pneumatolytic emanations cannot be 

 exactly defined, but it is certain that they consisted of water with soda 

 and silica in solution ; probably also carbonic acid was abundant, and 

 many other substances may have been present. 



7. In the British Isles spilitic eruptions have appeared in great 

 numbers in all the Palaeozoic formations (with the exception of the 

 Upper Silurian and the Permian), and in the Tayvallich Volcanic 

 Series have an important development among the metamorphic schists 

 of the Scottish Highlands. 



8. Like the Atlantic and Pacific igneous suites they have an 

 intimate connection with certain types of geographical conditions. 

 They are essentially rocks of districts that have undergone a long- 

 continued and gentle subsidence, with few or slight upward move- 

 ments, and no important folding. 



Bibliography. 



(1) Whitley, N., "On the Kemains of Ancient Volcanoes on the North Coast 



of Cornwall in the Parish of St. Minver " : Thirtieth Ann. Hep. Roy. 

 Inst. Corn., p. 60, 1849. 



(2) De la Beche, Sir H., Geological Observer, p. 585, fig. 223, 1853. 



(3) Teall, J. J. H., "On Greenstones associated with Radiolarian Cherts" : 



Trans. Roy. Geol. Soc. Cornwall, vol. xi, p. 560, 1895. 



(4) Raisin, Miss C. A., " Variolite of Lleyn and Associated Volcanic Rocks " : 



Q.J.G.S., vol. xliv, pp. 145-65, 1908. 



(5) Reid, C, & Dewey, H., " The Origin of the Pillow-lava near Port Isaac, 



in Cornwall" : Q.J.G.S., vol. lxiv, pp. 264-72, 1908. 



(6) Geikie, Sir A., "The Geology of Central and Western Fife": Mem. 



Geol. Surv., p. 54,1900. 



(7) Brongniart, Al., Class, et caract. mm. des roches, 1S27, p. 98. 



(8) Bonney, T. G., "On the so-called Spilites of Jersey": Geol. Mag., 



1893, p. 59. 



(9) Teall, J. J. H., "The Silurian Rocks of Britain"; vol. i, Scotland: 



Mem. Geol. Surv., 1899, pp. 84-7. 



(10) Bailey, E. B., & Grabham, W. G., "Albitization of Basic Plagioclase 



Felspars" : Geol. Mag., 1909, p. 250. 



(11) Termier, P., " Sur l'elimination de la chaux par metasomatose dans les 



roches eruptives basiques de la region du Pelvoux " : Bull. Soc. Geol. 

 France, tome xxvi, p. 165, 1898. 



