Academy of Scieistce. 



Standing in tliis semi-circle, as we turn toward the left the Causeway proper is 

 before us, and unlike most of the ^reat wonders of the world, it breaks upon the 

 view without disappointment. Here is a sharp promontory three hundred feet 

 high, cut down abruptly at the point, nearly to the level of the sea. This promon- 

 tory is composed of columns like those of the Causeway, but less perfect and 

 more irregular in arrangement. At the base of this cliff a break in the columns is 

 called the Giant's Gateway. This break forms the base of a triangle whose two 

 sides meet at a distance of one hundred and twenty- five yards at the water 

 level. These lines are the boundaries of the Causeway. As we stand on the base 

 of the triangle then, we are twenty-five feet above the water, ami have the whole 

 Causeway before us in one view. Here are more than forty thousand columns of 

 gray basalt standing in such close order that in most cases a knife-blade cannot be 

 inserted between them, — all so placed that the general slope from the base of the 

 cliff to the level of the sea is as gradual and smooth as if formed for an actual high- 

 way. The length of the whole triangle is, at low tide, 125 yards. 



We are told that, in all these forty thousand columns, " there is only one that 

 is triangular — it stands in the east side of the grand Causeway. There are but 

 three columns of nine sides, one in the honeycomb, and the other two near the 

 triangular pillar. The total number of four and eight sides bears but a small pro- 

 portion to the entire mass of pillars, of which it may be safely computed that 

 ninety-nine out of a hundred have five, six or seven sides." Some are almost 

 perfectly round, in others the angles of the face planes are extremely sharp and 

 clean cut — in fact, most of the columns are beautifully defined in their sides and 

 angles. The diameters of the pillars vary from eight to twenty-two iuciies — the 

 average is about seventeen. The heads of the columns over which we walk are all 

 either convexor concave. The concavity of a common sized column, eighteen inches 

 in diameter, will hold nearly a pint of water— the cavities are, therefore, shallow. 

 The tops of nearly all the columns are covered with a deposit of slime, which being 

 often wetted by the sea, and dried by the sun, gives a dark and old appearance to 

 the Causeway. This tendenc}' to break in concave and convex forms proves a 

 concretionary structure, and shows that each column is entirely separate from every 

 other. 



Dana states that the term basalt was early applied to a group of three rocks, viz: 

 Melaphyre, Doleryte and Peridotyte, belonging to the series Hornblende and 

 Pyroxene. Melaphyre and Peridotyte have nearly tlie same elements, texture and 

 specific gravity as Doleryte, so we take the composition of the latter as that of the 

 rock composing the Causeway. With a specific gravity of 2.75, there are as 

 elements: Silica, 48.00; Alumina, 16.28-, Protoxide of Iron, 15.55; Lime, 9.50; 

 Maixnesia, 3.85 ; Potash, 2.01 ; Soda, 2.01 ; Water, a fraction. A more popular 

 analysis would be to say these pillars are composed of about one-half flinty earth, 

 one-quarter iron, and one-quarter clay and lime. In rocks of concretionary 

 structure like basalt, the tendency of the melted material is to concrete about 

 centers. "Basaltic columns, then, are a result of this concretionary structure" 

 (this grouping about centers) "and each column corresponds to a separate action. 

 The size of the columns is determined by the consistency of the mass to be cooled ; 

 the thicker the mass, the slower the cooling and the larger the columns. The 

 cracks separating the columns are due to contraction on cooling. 



Such is the situation and the mechanical and chemical composition of the Giant's 

 Causeway ; but as we stand upon it and contemplate its wonderful formation, the 

 mind is flooded with such questions as these : How far does it extend under the 

 land and how far under the sea ? How and when, and under what conditions, did 



