644 



PHILOSOPHICAL TRANSACTIONS. 



[anno 1790. 



nous hue. Some veins of lead ore are also mentioned, but reported to be not 

 worth the working. 



If it be admitted, Mr. M. says, that he is right in his opinion of the volcanic 

 origin of these different substances, a large tract will then be added to that al- 

 ready proved by others to have been subject to the effects produced by subterra- 

 neous fire; which, as far as has hitherto been discovered with us, commences in 

 the s. w. part of Derbyshire, and is again seen in Seathwaite, about 5 miles from 

 Hawkshead, in the n. w. part of Lancashire, and appears, n. w. from thence, 

 in the neighbourhood of Belfast in Ireland, and ranging through the northern 

 part of that kingdom; it is perceived in several of the western islands of Scot- 

 land, extending as far north as the island of Lewis, which is the northernmost of 

 the Hebrides, and crossing east from Hay, which is the southernmost, by Tarbut, 

 Dumbarton, Stirling, and Edinburgh to Dunbar. Some persons may consider, 

 with astonishment, the extent of those veins and masses of lava which appear in 

 the northern part of the British isles, where no crater is visible; while others, 

 who have read Von Troil, and recollect that he says, " That lava is seldom found 

 near the opening of a volcano, but rather tuff, or loose ashes and grit," may per- 

 haps unite with Mr. M. in opinion with Mr. Whitehurst, " that the crater 

 whence that melted matter flowed, together with an immense tract of land to- 

 wards the north, have been absolutely sunk and swallowed into the earth, at 

 some remote period of time, and became the bottom of the Atlantic Ocean. A 

 period indeed much beyond the reach of any historical monument, or even of 

 tradition itself." 



The more readily to compare the specific gravities of the Hay lavas, and other 

 substances, mentioned in this paper, with those from other parts, a table of their 

 several weights is here added. 



Ardlun coal 1.284 



Jet, according to Dr. Watson 1 .236 



1.180 



Cannel coal from Haig in Lancashire . . 1.275 



Whyndyke, fr. near M' Arthur's head, N°l, 2.863 



Whyn dyke from Freeport, inside N° 2 . . 2.881 



The same from outside, N° 3 2.850 



Whyn dyke from Gartness, N° 4 2.63 1 



5 2.698 



6 2.484 



7 2.342 



8 2.322 



Whyn dyke from Gartness, N° 9 2.833 



10 2.652 



Basaltes from the Giant's Causeway 2.743 



from Fairhead 2.950 



from Ardlun 2.724 



from StafFa 2.736 



Vitrescent substance from Ardlun 2.800 



Toadstone, from great rocks, I r qq 



dark compact 2.634 



ditto cellular 2.528 



yellow grey from Bonsai 2.219 



Derbyshire, yellow grey 



Fig. 14, pi. 6, is a view of the glen near Ardlun Head in Mull. 



Fig. 15, a view of the insulated rock at the termination of the glen. 



Fig. 16, a view of the great fissure, the cave, and the suspended stones, in the island of Mull. 

 The fissure ranges n. and s. is about 10 feet wide and 40 yards deep j the sides and the suspended 

 stones are gi;anite. 



