^8 



Me, and the Dyke proceeds now due fouth : all this is beft explained 

 by a figure. 



N. N. E - . S. S. W. 



The Dyke, after having proceeded a fliort way in its new direftion, 

 is loft under the rubble that has fallen from above ; but whenever the 

 precipice becomes perpendicular, it appears again in its iaft direftion, 

 cutting the ftrata vertically from the bottom of the precipice to the top, 

 above two hundred feet ; the height of the upper_ part of the cliff 

 above the fca is here three hundred and thirty feet. 



Thefe ftrata are almoft all columnar, and the horizontal prifms of the 

 Dyke are ftrongly contrafted with the vertical pillars of the ftrata. 



The bafalt of this Dyke is very nearly of the fame grain with that 

 of the Dyke at the Caufcway, rather coarfer, its frafture granular, and 

 full of fliining points ; but it differs materially from it in another refpeft, 

 the latter having but one principle of conftruftion, to wit, the minute prifms 

 into which it breaks, and the agglutination of thefe forming it into a mere 

 wall ; while the Dyke at Fort Spagna has, like fome other varieties of 

 our bafalt, a double principle of conftruftion, being firft formed into huge 

 maffive prifms four and five feet in diameter, and thefe again being divided 

 into fmall quadrangular prifms whofe fides do not exceed an inch. 



This property pofTeffed by fome varieties of our bafalt, and other 

 curious circuraftances attending them, as for inftance, that fome of our 

 prifmatic bafalt in thin ftrata, abound with marine exuviiE, fliells and 

 imprefiions of cornua ammonis,* while others, columnar and prifmatic, 



but 



* The nature of this (lone is, I know, not yet fully afcertaincd. Sir Jofeph Banks 

 informs me, that the fpecimens I i'ent to him, are pronounced by his friends not to be 



^eiiiiiiit: 



