34 J. Lomas — Quartz Dykes near Foxdale, Isle of Man. 



should perhaps forbid us laying much stress on its absence in this 

 particular locality. 



On the evidence of the Faringdon Belemnites, then, we may 

 conclude that the ' Sponge-gravels ' are probably equivalent to the 

 lowermost portion of the Lower Greensand Series of south-eastern 

 England ; and I am not aware of any facts adverse to this conclusion 

 either in the other components of the fauna or in the stratigraphical 

 position of the deposit. I hope hereafter to discuss the relations of 

 the Faringdon Beds to other fossiliferous portions of the Lower 

 Greensand of the Midland counties. 



VIIL — Quartz Dykes near Foxdale, Isle of Man.^ 

 By J. Lomas, A.E.C.S., F.G.S. 



IN the neighbourhood of Foxdale, Isle of Man, and especially on 

 Granite Mountain, the ground is strewn with numerous blocks 

 of quartz. Many of thera are of large size, 10 feet or more in 

 diameter. 



On the slopes of South Barrule similar blocks are found in great 

 profusion, and they can be traced across the hills to the west coast. 

 North of Foxdale other bands are found, some of which are inserted 

 in the geological map. They lie principally in the altered slates of 

 the Barrule Series, and have a general trend corresponding with the 

 main axis of the island, from north-east to south-west. In places 

 they are seen to be in situ, and where the granite mass of Foxdale 

 intervenes the general direction changes, and is principally tangential 

 to the intrusion. 



Numerous micro-granite dykes extend along the axis of the 

 island in the same direction. 



In an old quarry at Eenshent on the north margin of the granite 

 several quartz veins are seen to traverse the granite itself. They 

 can be traced along the floor of the quarry and up the vertical face 

 about 30 feet high. 



One of these, about 3 feet in width, shows perfectly sharp margins 

 when cutting through the granite, dips at 65° W., and strikes 

 10° E. of S. It consists mainly of quartz, some clear and some 

 white and opaque, but on entering the granite it changes locally to 

 a pegmatite. The pegmatite contains, in addition to the quartz, 

 large felspars, some over 3 inches long, perfectly formed, and 

 showing crystal faces, and mica in crystals over an inch in diameter. 

 The granite outside the vein shows the normal features of the 

 medium-grained Foxdale type. 



Under the microscope the clear quartz of the vein is seen to 

 consist of one crystal, all parts of the field extinguishing at the 

 same angle. The white opaque parts behave optically in the same 

 manner, but numerous bubbles containing liquid are contained, and 

 these, no doubt, give rise to the opacity. The inclusions are mostly 

 in lines, and sometimes take the form of negative crystals, the 

 principal axes of which are parallel to the cross wires when extinction 

 takes place. 



1 Eead before the Liverpool Geological Society, November 11th, 1902. 



