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



Description of the Map. (Plate XVI.) 



The map is on the scale of 4 inches to 1 mile, and shows the shape 

 of the outcrop of the granite mass. At the points marked J actual 

 junctions between the granite and the altered country rock are exposed. 

 Elsewhere the boundary is lined or dotted according as to whether the 

 mapping is certain or only probable. 



In the quarry below the Foxdale lead-mines (see Map, " slate, grit, 

 and granite quarry") a good junction is seen. Between the granite 

 and the slate a thick vertical strip of finer-grained granite occurs, 

 which is continuous with a fine micrograuite dyke on the other side of 

 the quarry. 



Tracing the boundary south it can be seen to cross the stream 

 (Struan Barrule) in several places and then to sweep east up the hill 

 till we come to the apophyses marked on the map. These apophyses 

 can be easily traced round the slate quarry, and at J is the slickensided 

 junction with a dip of 30° (approx.) to the south-east under the 

 slates, described in the Survey memoir. The granite of the apophyses 

 is very fine-grained, or rather, contains strips of fine-grained micro- 

 granite. Further east the boundary is somewhat obscured by bog, 

 but is fairly clear as one approaches the farmstead of Benshent, where 

 a junction with highly altered slates is seen in a little cutting. Then 

 no other junctions are seen till the Foxdale Silica Quarry is reached 

 (see Section, Fig. 2, p. 349), north of which a small apophysis runs 

 out from the granite (Survey memoir, p. 165). 



So far we are more or less in agreement with the Survey 1 inch map, 

 but on this map the quartz veins are not shown. There may be other 

 veins besides those shown upon the accompanying map, as there is 

 considerable peat and boulder-drift, but the main veins are indicated 

 as far as they can be traced. It will be noticed that these veins lie 

 in three sets, somewhat radial, or approximately in two directions, viz., 

 north and south, east and west. In the Foxdale Silica Quarry (Fig. 2, 

 p. 349) — "the Spar Quarry" — there are several thick veins to be later 

 described. The veins occur in greatest number to the south of the 

 intrusion and several of them are 12 feet thick, while one for a short 

 distance attains a thickness of 18 feet, but soon thins out to 2 or 3 feet 

 on either side. Opposite Benshent (see Map, Plate XVI) a vein 12 feet 

 in maximum thickness can be traced some way up the 'mountain', and 

 it seems as though it is continuous with the vein outcropping in the 

 stream to the east, but there is boggy ground between. The Eairy 

 granite mass to the east is a smaller mass in every way similar to the 

 main outcrop. Between the two outcrops the slates and grits are much 

 altered and are exposed in the little stream. In the Survey memoir 

 the question is raised as to whether this Eairy mass is connected with 

 the Foxdale mass below or as to whether it is merely a portion of the 

 latter that has been thrust into its present position. Since the Survey 

 visited the spot a section of the junction of the granite with the slate 

 has been exposed by quarrying in the Eairy mass for silica, and there 

 seems evidence here that the mass is in situ and is a similar and 

 connected intrusion. (See description of Section, Fig. 3, in Eairy 

 Silica Quarry.) 



