2B^ ORIGIN OP BASALT. 



Facts and ob- tli'S class are, the columnar, the irregular prismatic, and 

 servations re- the tabular. 1 have not been able to discover subor- 

 specuns basalt i- , r • ^ i • , • • c .^ 



in the county "'fate lorms, or an internal construction, in any or these 



of Antrim; ad. basalts. 



that^it irnot"^ Other varieties, on the contrary, are regularly arranged 

 volcanic. internally ; the large prism breaking into smaller, some- 



times to a great degree of minuteness, as in the Portrush 

 silecious basalt. The coarse Portrush Basalt, whose prisms 

 are mostly quadrangular, and the unarticulated pillars of 

 Ballylagan, have likewise the same property, in an inferior 

 degree; while the basalis of our whyn dykes have often their 

 subordinate prisms finished with great neatness.* 



But the forms into which our basaltic masses divide, are, 

 by no means, limited to prismatic alone. The pyramid 

 is a common figure in our whyn dykes ; and the most per- 

 fect joints of the Giant's Causeway pillars, partake both 

 of the prism and of the pyramid, and have also a mix- 

 ture of curve, and plain surfaces : the latter in number 

 equal to the denominator of the figure ; while the former 

 amounts to double that number, plus two. Thus, a pen- 

 tagon joint, taken from one of our most perfect pillars^ 

 has five plain, and twelve curve surfaces ; but curve sur- 

 faces are irreconcileable, either to crystallizatioti or desic' 

 cation . f 



We 



* This subordinate construction is well illustrated, in a drawing of 

 three prismatic stones, taken from a great whyn dyke, now \ised as a 

 quarry, nearly two miles west from Belfast. 



The constituent figure here is a triangular prism, whose angles, at 

 the base, seem double the angle at the vertex. 



My ingenious friend, Dr. M'Donnel, to whom I had mentioned the 

 curious construction of onr whyn dykes, was so struck when he saw 

 the prismatic stones of which this dyke is formed, extracted frOm 

 the quarry, that he employed a painter to make a drawing of some of 

 them ; and he was so good as to give me a copy. 



f The acute angled triangular pyramids, which ascend from each 

 angle of the joint, and often reach up to the middle of the incum- 

 bent one, have their insides sloped away, in an hyperbolic curve; 

 ' while the grooves in the lower part of each joint adapted to receive 

 these, with similar curvature, added to the former, make twice as 

 many curve surfaces as the figure has angles. The concave and con- 

 vex 



