2^ 



where they arc feen cutting vertically the fevcral ftrata of which thefe 

 are cojnpofed, and then burying themfelves in the northern ocean. 



The obfervations made on thefe Whynn Dykes in the two countries, 

 taken together, make our information on the fubjeft complete. In the 

 Hebrides we arc furprized at the incredible length to which thefe mighty 

 walls proceed, and we fee them penetrating indifferently all fubftances they 

 encounter : with us we can meafure a part, and a part only, of their 

 (tupendous height, as at the Mileftone one hundred feet, at Port Spagna 

 three hundred and thirty, at Fairhead probably more : and we can obferve 

 the effeft, or rather the non-effeft, produced at their contafts with the dif- 

 ferent materials they meet, as they are feen in the faces of our precipices. 



By Mr. Mills's account, (Phil. Tranf. 1790.) the ifland of Lifmorc, 

 entirely limeftone, is croiled by Whynn Dykes, as is the limeftone at 

 Gartnefs; at lona granite is the contiguous matter, at Juva chert, at 

 Perfabus a Whynn Dyke is croffed by a lead vein, and another at Glaf- 

 cow Beg ; at the ifle of Arran Mr. Jamefon finds them cutting through 

 porphyry and micaceous ftiiftus. 



With us the Whynn Dykes at the weflward of the Giant's Caufeway 

 cut through ftrata of table bafalt, and red ochreous matter, placed alter- 

 nately; at the Giant's Caufeway, and Port Spagna, they cut through ftrata 

 of finer bafalt, dispofed in prifmatic pillars ; while at Fairhead they en- 

 counter new materials, to wit, alternate ftrata of freeftone and coal. 



In both countries thefe mighty walls are always of bafalt ; their gene- 

 ral thicknefs is from twelve to fifteen feet, though in one or two inftances 

 they do not exceed two or three feet, and at Gartnefs the Whynn Dyke 

 is twenty three yards acrofs ; but it has not been afcertained in any in- 

 ftance to what depth they reach beneath the furface, even in the deepeft 

 mines. 



Though the material of which thefe walls are compofed feems to be 

 in general the fame, yet from Mr. Mills's account, there are important 

 differences between the Scotch Whj-nn Dykes, and with us fcarce any two 

 of our Dykes, that are acceffible, exaftly (as will appear) refcmble each 

 other. 



As 



