VOL. LXXX.] PHILOSOPHICAL TRANSACTIONS. ' 643 



feet wide, and, by estimation, about 120 feet deep, At tlie northern extremity, 

 near the top, 2 stones are suspended in a most extraordinary manner between 

 the sides: the under one is fixed, and on that the other appears to lie loose. 

 (See fig. 16). There is a large cavern in the western side of the fissure, and a 

 corresponding fissure is seen on the opposite shore. 



Having shown that whyn dykes, or in other words veins of lava, are found in 

 the vicinity of columnar basaltes, which latter are now, by almost universal con- 

 sent, acknowledged to be of volcanic origin ; Mr. M. now proceeds to describe 

 the whyn dykes of Hay. Hay, from the northern to the southern extremes, is 

 about 30 miles in length, and in one part extends nearly as much in breadth 

 from the eastern to the western shores. After a rather minute account of several 

 parts of the island, Mr. M. continues: The whyn dykes are too singular in 

 their formation to escape the eye of the naturalist who traverses this island: they 

 are masses, or rather veins, generally of a dark brown, apparently basaltic, 

 matter, not unfrequently containing bladder- holes; from 3, 4, and 6 feet, to 8 

 or more yards in breadth, running in various directions. In some places they are 

 straight for a considerable length; in others, their course, though progressive, 

 is inflected; and in some parts they rise between 3 and 4 feet above the surface, 

 forming natural boundaries or dykes, standing vertically, and appearing to fill up 

 the chasms formed at some remote period in the strata. This is instanced in 

 several of these dykes, in diflferent parts of the island. One in particular stands 

 vertically, is many yards in height, projects from the cliffs to the north-westward, 

 and in that direction runs many fathoms into the sea. It bears the buffeting of 

 the waves of the Atlantic Ocean from the south-west, and seems to defy their 

 rage, though its breadth, compared with its height and length, is very inconsi- 

 derable, being not more than 5 or 6 yards wide. It is of a dark granular sub- 

 stance, very similar to the whyn dyke near Freeport, excepting that the central 

 part is softer and of a paler colour. The outer sides, which are each about 2 feet 

 thick, are of a very dark colour, hard, contain some bladder- holes and specks 

 of zeolite, are generally detached from the centre by very small joints, and the 

 whole is divided by transverse joints into irregular polygons cf various dimensions. 

 If this stupendous object is viewed from the north, it has much the appearance 

 of a lofty wall of human fabrication. " A small distance more to the southward 

 is the great cave, in the Erse dialect called Ea mawr. The entrance is near 23 

 yards wide, and from 6 to 8 yards high. After going in a little way the roof rises, 

 and the cavern extends in breadth; but at about 150 yards from the entrance, all 

 its dimensions are contracted, and it becomes so small as barely to admit further 

 progress by crawling on hands and knees. There are some calcareous stalactites 

 pendent from the roof; and in this cave, as well as several others, wherever the 

 water pervades through the joints of the chert, it tinges the sides of a ferrugi- 



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