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



The granite contains plagioclase and microcline as well as a little 

 orthoclase locally. The dominant mica is muscovite. The order of 

 crystallization of the constituents is as follows : — 



Muscovite. 



Plagioclase (chiefly albite) . 



Orthoclase (when present) . 



Quartz. 



Microcline. 



The analysis shows the rock to be of a normal soda-granite type 

 with a high silica percentage. The results compare fairly well with 

 the analyses of granites of the Lake District and Ireland, except that 

 aluminium is in higher percentage and the alkalis a little lower. 



The condition of affairs seems to have been this — the main mass of 

 the magma cooled to form the acid soda granite of which the analysis 

 is given, but at the last there was a residuum of silica which 

 crystallized out to form the peculiar quartz veins which run through 

 the granite. The veins are igneous in origin, and there is no sign 

 of 'comb' or other structure to suggest that water or vapours have 

 helped in the formation. 



Dendritic Markings in the Microgranite Dykes. 



On Windy Common, about two miles south of the ' Granite Mountain', 

 microgranite dykes occur with also a few quartz ' dykes '. The latter 

 run north and south, but the former, of which seven could be mapped, 

 all run north-east to south-west. Several of these 'elvans' have 

 been quarried, and some are still being worked. In one of the most 

 actively worked dykes peculiar dendritic markings occur. They were 

 first noticed by Professor Boyd Dawkins, who kindly directed me to 

 the locality and placed his specimens at my disposal. The dendritic 

 markings are solid — not marks on one plane on the surface of the rock, 

 but marks due to the actual soakage of solutions depositing oxides 

 of iron in the rock. They always start on a joint plane, the sides of 

 which are stained with the iron oxide, and run varying distances into 

 the microgranite. Broken surfaces parallel to the joint plane give 

 numerous circular dark marks, and sections at right angles to the 

 joint planes give irregular marks, sometimes dendritic, sometimes 

 almost straight. In section under the microscope the rock is 

 seen to be a microgranite with quartz, felspar (plagioclase and 

 orthoclase), and muscovite. The mica usually exhibits a tendency 

 to parallelism and other evidence of pressure is not wanting. The 

 staining in section is seen to be merely infiltered colouring matter 

 between the grains. The dendritic marks can be wholly dissolved out 

 of pieces of the rock by hydrochloric acid, yielding a yellow solution 

 of ferric oxide and the fragments becoming quite white. As stated 

 above, the dendrites occur also at Eairy Silica Quarry, and here also 

 one gets the typical surface markings, properly dendritic in outline. 

 Specimens of the dendritic microgranite are very striking and the 

 usual character of the rock is hidden by the dark staining. It is 

 a peculiar infiltration effect which seemed worthy of notice in 

 this place. 



