IS FINGAL'S CAVE ARTIFICIAL? 235 



occur on the south {sic, query west) and the north sides are remark- 

 able neither for beauty nor for magnitude." The caves are sufficiently 

 numerous to furnish an argument. There are very few hollows worn 

 by the sea in the Scotch coast. The islet, which contains a dozen, has 

 not the yooWd P ar * f * ue indented line of the mainland, and bears an 

 infinitesimal ratio to the sea-board, including the islands. Its parent, 

 Mull, within whose bosom rests this irregularly oval rock, " measuring 

 about one and a half mile in circumference," has in the dimension of 

 length one hundred and fifty times better right to a " museum of won- 

 ders." The "Isle of Columns" is a speck too tiny to show on any 

 ordinary map. The chance that it would contain, as a legitimate yet 

 exceptional result of normal contact between igneous rock and sea- 

 water agitated by wind, "the most remarkable cave in Europe," is less 

 than 0. It is the V 1. 



The uneven table-land is formed of " three distinct beds of rock of 

 unequal thickness, inclined toward the east at an angle of about nine 

 degrees. The lowest is a rude trap tufa, the middle one is divided 

 into columns placed vertically to the plane of the bed, and the upper- 

 most is an irregular mixture of small columns and shapeless rock." 

 The columnar bed- is never more than GO feet thick. The island itself 

 attains a maximum height of 129 feet. It has no peak from which 

 rain-water might descend in a considerable quantity. There is no 

 series of fissures corresponding to the perforations. There is nothing 

 on the flat top to suggest the tunnels beneath. Proceeding toward 

 the south from the landing-place, there are six cases of alleged erosion, 

 each presenting its own seemingly insuperable difficulty, and cumula- 

 tively requiring a more thoughtful and serious consideration than the 

 fantastic phrases in which 'stupendous (!) columns, three feet thick 

 and thirty feet high, rise from a dark-red or violet-colored rock over 

 which the ocean rolls, and reflects from its white bottom a variety of 

 crimson and yellow." 1 



It appears now to be well established that the peculiar structure 

 of columnar basalt is due to contraction and splitting consequent upon 

 cooling. The analogy is rather to the splitting often seen in the mud 

 bottom of a dried-up pool than to ordinary crystallization. The vari- 

 ous conditions point to the contractile origin of the structure, at the 

 same time that the result suggests a curious mimicry of imperfect 

 crystallization. If the cooling mass of basalt be in one of its vertical 

 sections of such a form that successive isothermal couches, taken in 

 descending order, are not parallel to the original cooling surface, 

 as they are in all cases of straight and parallel prisms, but divergent 

 gradually from the cooling surface and from each other, then the lines of 

 the splitting of the prisms, always true to these couches, must be curved 

 in one direction. This will be true, whether the isothermal couches be 

 plane surfaces divergent from a thinner to a thicker part of the mass, 

 or whether they be curved surfaces arising from the mass reposing on 



