W. C. Simmons — Granite of Foxdale, Isle of Man. 345 



calcite, in the rocks described by him, have arisen by the alteration of 

 silicates under the influence of carbon dioxide. 



It seems equally clear, however, that the process of dedolomitization 

 is not quite so simple as has been supposed, and its explanation 

 seems to call for still more facts. 



III. — The Granite Mass of Foxdale, Isle of Man ; -with some 

 notes on Dendritic Markings in Microgranite Dykes. 



By William C. Simmons, B.Sc, A.R.C.S. 

 (MAP, PLATE XVI.) 



THE mining village of Foxdale, situate a little towards the southern 

 end of the north-east to south-west central trend-line of the Isle 

 of Man, lies just north of the barren heath-covered hill called " Stoney 

 Mountain ". On the 6 inch Ordnance Survey Map the northern part 

 of this hill is called Granite Mountain and the southern portion 

 Windy Common. The whole forms a long, regularly sloped hill with 

 its greatest length — about 2 miles — approximately north and south. 

 Though of small altitude its singular barrenness makes it conspicuous 

 in a moderately well-cultivated district. To the west South Barrule 

 and the Barrule Slate Quarries rise considerably above it, though on 

 the other sides the ground slopes gently off to more distant hills. 



The granite occupies only about a third of the whole area of Stoney 

 Mountain, and that all at the northern extremity. A small isolated 

 exposure of the granite occurs a short distance to the east, near the 

 little village of Eairy. These granite masses are intrusive into 

 the Manx Slate Series, which have been in places considerably altered. 



In both the larger Foxdale granite mass and the latter smaller one 

 thick veins of quartz and coarse pegmatite occur. Sometimes these 

 quartz veins, then resembling dykes, are found outside the granite 

 intruded into the country rock. Lastly there are the characteristic 

 microgranite dykes, associated with the granite mass and usually 

 intruded in the slates with a N.E.-S.W. trend. 



The field relations of the " Granite and its Elvans" are discussed in 

 the Survey memoir of the Isle of Man (G. "W. Lamplugh, 1903), to 

 which also notes on the petrology of the rocks by Professor Watts are 

 appended. Much quarrying has since been carried on in the quartz 

 veins for silica, and consequently better sections can now be seen. 

 It seems that a few further notes upon these very peculiar ' quartz 

 dykes ' may not be out of place. 



Mr. Lomas ' has described some of the veins and gives a rough 

 description of their distribution, arriving at the conclusion that both 

 those in the granite and those in the country rock are of igneous 

 origin. Beyond brief notices in the Survey memoir and elsewhere, 

 and. a short description by Mr. A. Harker, 2 little other literature on the 

 matter has come to hand. 



1 Lomas, Geol. Mag., 1903, pp. 34-6. 



2 A. Harker, Naturalist (Leeds), March, 1894 (note). Id., " Grainsgill 

 Greisen": Q.J.G.S., vol. li, p. 143. 



