Account of the Whynn Dykes, ^c* 365 - 



and we. see iheni penetrating indifFerently all substances 

 ihey encounter: — with us we can measure a part, and a part i 

 only, of their stupendous height, as at the Milestone lOOw 

 feet, at I'ort.Spagn.a 330 feet, at Fairhead. probably nioreji. 

 and we can observe the eifect, or rather the non-effect, », 

 produced at their contacts with the different niaierials they 

 meet, as they are seen in the faces of our precipices. 



By Mr. Mills's account, (Phil. Trans. 1790,) the islatii^c; 

 of Listnore, entirely Jimestoue, is crossed by whynn dykes, ' 

 as. is the, limestone at Gartness; at lona granite is the con- 

 tiguous matter, at Juva chert, at Persal)us a whynn dyke 

 is crossed by a lead vein, and another at Glascow Beg ; at 

 the isle of Arran Mr. Jameson finds them cutting through 

 porphyry and micaceous schistus. 



With us the whynn dykes at the westward of -the Giant's 

 Causeway cut throuiih strata of table basalt, and red 

 ochreous matter, placed alternately; at the Giant's Cause- 

 way, and Port Spagna, they cut through strata of finer ba- 

 salt, disposed in prismatic pillars; while at Fairi^ead they 

 encounter new materials, to wit, alternate strata of free- ■ 

 fctone and coal. 



In both countries these mighty walls are always of ba- 

 salt; their general thickness is from twelve to fifteen feet, 

 though in one or two instances they do not exceed two or - 

 three l^^et, and at Gartness the whynn dyke is 23 yards -t 

 across ; but it has not been ascertained in any instance t0 :• 

 what .depth they reach beneath the surfacCy ^ven in the 

 deepest mines. 



Though the material of which these walls are composed 

 seems to be in general the same, yet fronj Mr. Mills's ac- . 

 count there are important diilbreuces between the ScotcHn 

 whynn dykes ; and with us scarce any two of our dykes, . 

 that are accessible, exactly (as will appear) resemble eack 

 other. 



As the whynn dykes Mr. Mills observed are unques- 

 tionably basalt, he calls them all lava, and attempts iQ^t 

 prove it by a sort of vague induction : page 75 he say^S/, .. 

 Islay whynn dykes resemble those at Ballycasile, which \ 

 take tilth rise in a ooiuitry confessedly alounding with vole 

 fmic matter, ■ • » 



No.w the specimens from the islay dykes strongly re-ij 

 semble (as he says) the Derbyshire toadstone, formed, as- • 

 he asserts (page 98)5 by subterraneous fire. 



Of Derbyshire I will not presume to say any thing, hav- : 

 ing, never visited it; but the proof of its strata being lava 

 rests upon the admission of Mr. Whitehursi's position,.] 



that 



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