Certain varieties of lava, especially that which is found in dikes, form cool, beautiful columns called basaltic columns. They are due to the contraction that occurs on the cooling of the material. Instances of basaltic columns are seen in the Giant's Causeway, on the northern coast of Ireland, as well as in the Isle of Cyclops on the coast of Italy. The general appearance of the latter is represented in [Fig. 30].
Fig. 30. Basaltic Columns, Isle of Cyclops, Italy
It is a curious fact that the entire mass of basalt does not generally take the columnous form but only certain layers which terminate suddenly above and below at structureless masses of basalt, as shown in [Fig. 31]. These columns, however, are always found at right angles to the cooling surfaces as seen in the figures. They may, therefore, be inclined at all angles to the horizon.
Fig. 31. Columnar and Non-Columnar Basalt
When molten lava is only thrown up a short distance into the air from a crater it is still partially molten when on falling it again reaches the earth, and therefore clings to any surface on which it falls. There are thus built up curious cones known as driblet cones, in which the separate drops covering the sides of the cone can be distinctly traced. Driblet cones are represented in [Figs. 32 and 33]. Here, as can be seen, the separate drops can be readily traced as they run down a short distance before cooling.