and County of Antrim. 251 



I must here observe, that wherever our author uses the word 

 Jissxired, he means the divisions into pillars or prisms ; that 

 is, the basaltic arrangement, common both to dykes and 

 facades. — He proceeds : " Resting on the summit of these hills, 

 we see huge detached masses of rock, which rise several hundred 

 feet above them:" again, " One observes here, besides the hori- 

 zontal and parallel strata, that they are all penetrated by huge 

 perpendicular strata of loose and fissured rock." 



" \\'\t\\ respect to the perpendicular strata — they are often of 

 great breadth, and all regularly fissured ; the fragments quite 

 separate and distinct; but as uniformly fashioned, and evenly 

 placed, as the stones of a building." 



" Several of these vertical strata rose considerably above the 

 plane of the hills which they penetrated, and presented the ap- 

 pearance of huge walls of stone, surmounting their summits, and 

 descending along their declivities to the base." 



There appears to be an exact similarity between the prismatic 

 ^tones of which the St. Helena and Antrim dykes are formed. 



" The fragments which compose them are of all sizes, some 

 of them being six or eight feet long, others only a few inches, 

 but so regular and smooth, that they seem well adapted to the 

 purposes of masonrv, without the aid of the chisel or hammer." 



I have stated the pillars of our Antrim colonnades to be formed 

 })y the accumulation (in a vertical direction) of prisms, exactly 

 .Mmilar ; but that these have no internal principle of construc- 

 tion, the great joint breaking irregularly, and v.'ith a conchoidal 

 fracture — while the great prisms of which our dykes are formed, 

 and laid as it were l)v a mason, in a horizontal position, have 

 a subordinate principle of construction, breaking, not like the 

 others, with a conchoidal fracture, but into smaller prisms, al- 

 ready formed, with their sides brown and polished; and I call, 

 for distinction, these two descriptions, component and consti- 

 tuent prisms, a style of construction peculiar to wliyu dykes, and 

 which extends also to tliose in St. Helena, as appears clearly from 

 the foregoinsr passage. 



Our author tells us " of masses of irregular rock cemented to- 

 gether with a ponderous lava." Nothiuo: commoner with us than 

 masses of sound ba'^altcs, cemented together by a sort of solid 

 basaltic mortar, the fracture of the former, blue ; of the latter 

 gray, and granular; such is the mass of rock upon which Dun- 

 i;i(e castle stands. 



It cannot be deemed extraordinary that districts formed by 

 Nature of the same materials, should have these materials simi- 

 larly arranged in each ; but where we find differences, we are 

 led to inquire whether they arise from a diversity in the original 

 formation; or are the result of posterior operations; and if 



we 



