J. Lomas — Quartz Dykes near Foxdale, Isle of Man. 35 



A little to tbe east of the quarry is a larger quartz vein about 

 20 feet wide, which has been worked, and good exposures are 

 visible. The quartz has exactly the same characters as that already 

 described, and the vein can be traced as a feature and by occasional 

 outcrops to Windy Common, above Cloughwilly, a distance of over a 

 mile. Here several cuttings have recently been made, which show 

 the large blocks on the surface to be in situ. One of these has 

 proved to be 30 feet wide ; it cuts through altered slate containing 

 numerous needles of epidote, and is accompanied on the east side by 

 a micro-granite dyke. 



Another cutting further to the west and almost on the summit of 

 the knoll known as Granite Mountain, reveals a vein of quartz, 

 8 feet wide and dipping 65° E., contained in walls of much altered 

 slate. 



At Whallag, under South Barrule, 4 miles south-west of Kenshent, 

 the ground is strewn with quartz blocks which lie in great profusion 

 on the moor. Across one of the larger masses a trench has been 

 cut to prove its extent, and it is found to be in situ, and forms part 

 of the series of veins which traverse the slates. 



A little gulley cut by a stream near the Farm Reasch reveals 

 other quartz veins of smaller extent, and a small micro-granite dyke 

 not shown on the geological map. 



Dealing first with the Windy Common vein above Cloughwilly, 

 in the cutting it is seen to consist of white opaque quartz with joints 

 running transversely, forming horizontal columns. The joints are 

 more numerous at the margins than in the middle, reminding us of 

 similar features seen in basalt dykes. On the broken faces white 

 mica is often found. Under the microscope it is seen to be indis- 

 tinguishable from the quartz contained in the pegmatite. The liquid 

 enclosures and negative crystals are perhaps more numerous, but all 

 the quartz of the slide is orientated in the same direction, and 

 extinction takes place in all parts at once. In this rock there is no 

 sign of shearing. 



The 8 ft. dyke on the summit of Granite Mountain differs only 

 from the one just described in exhibiting traces of incipient shearing. 

 There has been no displacement, but cracks divide the crystal 

 into lenticular areas, leaving the whole in optical continuity. The 

 lines of bubbles are quite independent of the cracks. 



At Whallag almost exactly the same features occur, and fragments 

 of altered slate up to 5 or 6 inches in diameter are sometimes included 

 in the quartz. 



Further to the south-west, and in the same general direction, the 

 slates of the coast, as at Fleshwick Bay, contain quartz bands. They 

 are folded and twisted with the slates, and often separated into 

 lenticular bands. A slice from one of these bands shows under the 

 microscope quartz of exactly the same character as those described, 

 but shearing has gone on to such an extent that actual displacement 

 has taken place. 



Along the lines of movement a fine quartz mosaic has been 

 formed, but the units which have moved are simple crystals. 



