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



mineral. These quartz veins, as at Grainsgill, traverse the other types 

 of rocks." : It is significant that here in the Skiddaw granite quartz- 

 mica pegmatite and small quartz veins occur. The north-eastern 

 trend-line of the Isle of Man strikes into the Lake District, and the 

 Manx slates have been long supposed to be comparable with the 

 Skiddaw slates. 



Pegmatite is seen in the Eairy Quarry and marked ' pegmatite ' on 

 •the diagram. Mr. Harker uses 'greisen' to denote a rock with 

 felspar in subordinate amount, with a rather coarse-textured mosaic of 

 quartz and the mica in moderately large flakes. No topaz or cassiterite 

 are present. No felspar has been found in the quartz rocks in Eairy 

 Quarry, but rocks consisting simply of a mosaic of quartz and mica 

 are present and here are included among the pegmatite. In only one 

 case was a pegmatite found with felspar. A block of large-grained 

 pegmatite with quartz, muscovite, and kaolinized felspar was found 

 near the Foxdale Silica Quarry, but not in situ. In the Eairy Quarry 

 the coarseness of grain of the quartz rocks varies considerably. Some 

 of the pegmatite is very coarse indeed, the quartz grains often too 

 large to trace. In some cases the quartz consists of a mosaic with 

 flakes of mica, and this may possibly correspond with Mr. Harker' s 

 'gi'eisen'. It should be added also that there is considerable 

 evidence of dislocation and disturbance in this Eairy Quarry. Besides 

 the linionite-filled joints, pressure effects are seen in the pegmatite, 

 and in one case a large lenticle of quartz covered with a shining film 

 of mica was found. The granite shows a tendency to foliation in thin 

 section. 



Mode of Origin of the Quartz Veins. 



One cannot help speculating upon the possible ways by which these 

 •characteristic pegmatite and quartz rocks can have been formed. The 

 veins of quartz occur, as seen, in the granite itself and also in the 

 surrounding country rock. As far away as Fleshwick Bay, near Port 

 Erin, on South Barrule and on Slieau Whaullian to the north-west, 

 veins of quartz are found in the slates. These last, however, do not 

 appear as dykes ; they follow the cleavage of the slates in some cases, 

 and in others cut obliquely across in strings and bands ; sometimes 

 they are in irregular lenticles. In Fleshwick Bay thick veins 

 following the cleavage occur in the grits and slates seen there ; the 

 grit near the quartz veins is very much hardened by silicification. 

 Here and on Slieau Whaullian sometimes the quartz veins occur 

 in highly contorted slates, and the veins follow the intricate folds 

 more or less closely. In these cases we have, it seems, water-formed 

 veins — veins which have been formed by infiltration. On South 

 Barrule both forms of quartz occur. 



In the case of the veins in the granite, and of those ' dykes ' of 

 quartz in the country rock, the mode of formation seems to be in 

 every way comparable with that of normal igneous rocks. Veins 

 of aplite that occur in association with many of the acid plutonic 

 rocks are often spoken of as representing the mother-liquor of the 

 molten magma after the greater part has crystallized. In the normal 



1 " Grainsgill Greisen " : Q.J.G.S., vol. li, pp. 143-4. 



