35 



pnt together as if to illudrate Prop. 7. Lib. 1 2tli Eucl. Elem. I Iiad 

 found two or three fmall triangular pyramids among the quadrangular 

 prifms at the Giant's Caufeway Dyke, but at the Belfafi; Dyke triangular 

 pyramids were the fole elementary figure. 



As the fliore in Belfafl: lough is low, there are but few opportunities 

 of examining the materials that come in contaft with the bafalt Dykes ; 

 in fixR I noticed but two, flratified clay and freeftone ; this clay is very 

 plentiful on the fliore and the adjacent country, it is arranged in very 

 thin horizontal ftrata, and when expofed to the air hardens almofl to 

 the confluence of (tone. 



At the contaft the bafalt and freeftone were flrongly united together, 

 and for two or three inches the bafalt had in fome fort acquired tlie 

 colour and grain of the fandflone ; I was particularly attentive to this 

 faft as Mr. Werner alledges the tranfition of bafalt into other flones, and 

 Mr. Jamefon found in Arran (pages 131 and 135) bafalt fometimes 

 mixed with, and at others penetrated by fandilone ; but on this occafiou 

 Doctor M'Donald by fome experiments, found that notwithftanding the 

 freeftone appearance, the ftone remained pure bafalt. 



The bafaltic area, from the north and eaft fides of which thefe fingular 

 walls diverge in fuch abundance, comprehends a confiderable part of 

 the county of Derry, and a much greater of the county of Antrim ; its 

 breadth varies from twenty to near thirty miles, and its length exceeds 

 thirty-five ; it feems compofed almoft exclufively of vaft and fteady bafalt 

 ftrata accumulated upon each other j in one place we count fixteen, in 

 others we conjefture more, efpecially at Magilligan rock, as we know 

 the bafalt to be twelve hundred feet deep there. This whole mafs refts 

 upon a vaft ftratum of white limeftone about two hundred feet thick, of 

 the fame extent with the bafaltic area, but difcoverable only at its peri- 

 phery, which extends above eighty miles. 



This mighty ftratum afcends to the fouthward, until its lower edge acquires 

 on the eaft fide an height of eight hundred feet, and on the weft at leaft 

 feventeen hundred 5 the country below the limeftone ftratum, and without 



E 2 it. 



