AND SPHEROIDAL STRUCTURE. 141
I shall begin by describing the various structures which may be observed in rocks, more especially volcanic, and then consider how far they can be explained by the theory of contraction, as stated by the above-named authors. The advantage of this method will be that we shall see more clearly what phenomena have to be explained. I shall consider the spheroidal structure apart from the others.
It is almost needless to remark that though columnar structure* appears to be most frequent in basalt, it is not confined to that rock ; I have myself seen it in trachyte (district of Mont Dor, Auvergne), pitchstone (Arran), fclstone (Cader Idris, &c), phonolitc (Eoche Sa- nadoirc &c, Auvergne). Xor is it confined to igneous rock. I have observed it in volcanic mud (beneath a bed of basalt in Tides well Dale, Derbyshire), in coal at contact with basalt from Ayrshire (Geological Museum of Edinburgh) and from Yorkshire (AVoodwardian Museum, Cambridge), in haematite iron-ore (ib. ; the columns are about J inch in diameter), and, though rather imperfect, in palagonite tuff (Iceland), and in a large quartz vein (Svolvaer, Lofoten Islands). Ice also, when kept for some time very near a temperature of 32° Fahrenheit, as it is during a slow thaw or in those singular caverns termed glacieres, in the Alps, also exhibits a beautifully regular columnar structure, which I can only attribute to a contraction of the mass, probably as it passes from the point of minimum den- sity to the melting-point f.
. Occasionally one set of parallel divisional planes is more strongly marked than the others, so that, while the majority of the columns retain the hexagonal type, an oblong form dominates, and a some- what platy or bedded aspect is given to the rock-mass. I remember observing this especially in a trachyte in the ravine of the cascade of the Dore, on the Pic dc Sancy ; and it is very conspicuous in the great mass of felsite which rises above Llyn-y-Gader (Cader Idris).
Columnar structure is rarely set up quite close to the exterior of the mass ; there is usually an interval, varying from a few inches to several yards ; this is either affected by some other form of jointing or is quite irregularly cracked.
Where the space intervening between the columnar part and the exterior is but small, as, for example, in " the Spindle," near St. Andrew's, Fife, and at Montiqucy quarry, near Balmuto in the same county, it may be concluded that the mass soon began to lose heat very uniformly. Hence this structure is more likely to occur in intrusive masses (especially when the lava cooled at some depth like the above) than in streams of subaerial lava. In the latter also the columns often approach nearer to the under than to the upper surface. Thus the lava-stream of Eoyat is rudely columnar almost close to its base at the grotto of the Tirtaine, as is the stream from the eastern side of the Puy de Gravenoir, where it is exposed in a quarry on the east of the road from Clermont-Ferrand to Beaumont, while the upper part, still some yards thick, exhibits a far less
- As it may be considered to be now agreed that the sides of the prisms are
normal to the surface of cooling, I shall not multiply instances to prove this, t See 'Nature,' vol. i. p. 481, vol. iii. p. 288.
