W. C. Simmons — Granite of Foxdale, Isle of Man. 347 



Quartz and pegmatite veins occur in this mass also. The boundaries, 

 except at the above-mentioned junction, are nowhere seen owing 

 to thick boulder-drift. The distribution of the heath and boulders 

 and the levels give the probable mapping. 



The Quartz Yeins. 

 The Foxdale Silica Quarry (see Map, Plate XVI, and Section, Fig. 2, 



p. 349). 



Judging from the surface outcrops of the pegmatite veins an observer 

 sees little to lead him to any other conclusion than that these consist 

 simply of walls of white quartz spangled in places with mica, which 

 project through the peat among the granite blocks and can be traced 

 for a considerable distance up the mountain. They seem on the 

 surface to be simple dykes ; but when exposed in section in the 

 quarries the apparent simplicity gives way to a complexity at first 

 confusing, but which a little consideration easily elucidates. In 

 the quarry the 'dykes' are seen to be thick, nearly upright veins 

 which send out offshoots into the granite, dipping up at a high angle. 

 The figure gives a diagrammatic representation of the facts (Fig. 2). 

 The quarry face is irregular, and in the sketch the whole is brought 

 to one plane. The main veins are the two upright ones to the right 

 of the figure. The larger does not reach up to the surface, and it 

 sends offshoots to the right which have a high dip into the quarry 

 face. The irregular surface is here largely due to irregularity in the 

 quarry face combined with this high dip. To the left are three veins 

 with a dip of approximately 45° to the west, and farther out a larger 

 vein is seen which includes parts of the granite. The whole of the 

 material between the veins is a rather coarse-grained muscovite granite. 

 The quartz locally passes into pegmatite, which consists essentially of 

 quartz with spangles and matted masses of pale-bronze coloured mica, 

 which is of the same type as the mica of the granite. 



Several interesting things are to be observed in this quarry. In 

 one case (not shown in the Figure) a vein occurs which is practically 

 upright and has on its sides a felted mass of pale-bronze mica between 

 it and the granite. This selvage is about half an inch in thickness 

 and could be traced for two or three yards. On the west side of the 

 quarry a vein occurs which can be seen running up the hill for some 

 distance (see Map). In a little cutting outside the quarry the whole 

 of the vein for a short distance has been cleaned out, and the side 

 walls remain perfectly smooth and upright. These walls consist of 

 granite with a coating of a mixture of quartz and orthoclase felspar, 

 half an inch and less in thickness. This is a peculiar selvage ; the 

 felspar is idiomorphic and the quartz more or less allotriomorphic 

 though with parallel growth, and it is quite water-clear. The selvages 

 are of importance in considering the origin of the quartz veins. 



Another interesting fact here is the occurrence of 'lochs' in parts of 

 the quartz veins. Large drusy cavities occur which are lined with good 

 quartz crystals, in continuity with the quartz of the vein, and usually 

 therefore there is a tendency to a parallel growth in them. Rarely 

 the crystals are water-clear, but usually they are milk-white, like 

 the veins themselves (due to inclusions, see Mr. Lomas' paper). These 



