December, igo6.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



607 



Some Notes on the 

 Giant's Causeway. 



By Edward A. Martin, F.G.S. 



The wonders of the Giant's Causeway had drawn 

 visitors to the Antrim coast long ere its geological im- 

 portance was realised, and it may safely be said, even 

 now, that not one in a hundred who visit it have 

 any thoughts beyond those of mere curiosity, or con- 

 sider for a moment how it came into existence, and 

 what it all means- 



Whitehurst, in 1 786, was one of the earliest to under- 

 stand the significance of its basaltic pillars, and his 

 idea is still the accepted one, viz., that the rocks are 

 the results of out-flows of lava from some volcanic 

 centre or centres now hidden beneath the sea. 



The Causeway proper is, of course, that portion 

 which extends northward from the basaltic cliffs in the 

 shape of a promontory towards the sea, beneath which 

 it loses itself. But to the geologist, the whole of the 

 five or six small bays to the east of it are full of interest, 

 in consequence of the changes in the structure of the 

 rocks, and the fact that regular columns of some kind 

 or other are found throughout the whole, whilst the 

 scenery tO' which they successively give rise is sufficient 

 to attract numberless persons to whom geological facts 

 are still as a sealed book. 



The chief attraction in the Causewav is the large 

 number of regular angular columns into which the la\a 

 has broken up. I use the words "broken up," ad- 

 visedly, since there is no reason to believe that 

 crystallisation has had anything to do with the forma- 

 tion of the columns. They have simply broken apart 

 from one another, owing to the basalt, of which they 



Fig. I.— The divisions between these columns are very clear and open. 



arc composed, re(|airing less room, so to speak, when 

 cold, tiian it did when it was a melted mass. 



.'\s one walks or climbs over the heads of some of the 

 columns, one cannot help asking oneself how far 

 below they extend. If one looks at others which have 

 been exposed in the cliffs, one can count safely on a 

 length of about 20 feet, below which they probablv 

 would be found to close in upon one another and form 

 a homogeneous mass of basalt, cracked and jointed 

 irregularly, perhaps, but showing no columnar structure 

 at all. Tlie whole thickness of the lower basalts has 

 been estimated as about 60 feet- 



As we see the Causew ay dipping beneath the sea, we 

 shall realise the possibility of the basaltic flow follow- 

 ing on, hidden from our view, until it joins that which 

 appears in the Isle of Mull, off the west coast of Scot- 

 land. I^rofessor Judd has stated that the Mull volcano 

 must have been 14,500 feet high. A lava flow from 

 such a volcano would be no small affair, and a historic 

 parallel may be found in the flow^ in Iceland, in 1783, 

 when it extended for 50 miles. 



When lava is in a fairly viscid condition, the mass 

 may contract on cooling without fracturing, but when 



Fig. 2— Tops of columns sha.vingtilt to the west. 



it is approaching more nearly to a solid condition, it 

 cannot, as a whole, continue to contract. It must 

 break up into^ smaller masses. When w-hat has been 

 called its "breaking strain" has been reached, the 

 rock will exhibit a series of layers of increasing tem- 

 peratures in a downward direction. In this way the 

 surface and that immediately underlying will exhibit a 

 " platy " structure, but below where this appearance 

 is exhibited, there will be a region of lava, whence the 

 heat IS but slowly being conveyed away, owing to its 

 l)ad powers of conductivity. Then a new^ set of strains 

 ap[x;ar. There must be contraction, and some part 

 must give way and be fractured. The tension becomes 

 greatest in a lateral direction, and fractures appear, 

 reaching down perpendicularly to the direction in which 

 the lava flowed. It is difficult, if not impossible, to say 

 what conditions are necessary for the fractures to ap- 

 pear so regularly as they have in the case of the Cause- 

 way, or why, in other cases close by, irregular blocks 

 with but a starch-like columnar structure only should 

 apix."ar. When the columns are regular, they are 

 generally hexagonal, and often the sides are remark- 

 aliiy equal in length. Why the hexagon should be by 

 lar the commonest form has been a matter of much 

 discussion. Professor Bonney states in this respect, 

 that Mr. Mallet has shown that " there are t)nly three 

 reguUxr forms into which plane space can be divided, 

 \ iz., equilateral triangles, squares, and hexagons, and 

 the amount of work which has to be done in producing 

 these is given approximately by the ratio, 100 : 6S : 52. 

 So the last of the three requires the least expendi- 

 ture of force." It is stated with probable truth that 

 99 per cent, of the columns in the Causewav are hexa- 

 gonal, and that only one triangular column is to be 

 found there. 



-Mthough in certain groups the sides of the columns 



