The hexagonal prisms or columns formed by contraction during the consolidation of lavas vary greatly in size, according to the rate of cooling, the nature of the materials, and the conditions affecting the mass. Sometimes such columns may be found having a diameter of eight or ten feet and a length of five hundred feet, as in the Shiant Isles lying to the north of the Island of Skye; in other cases, as in certain volcanic glasses, minute columns, an inch or two in length and scarcely thicker than a needle, are formed; and examples of almost every intermediate grade between these two extremes may sometimes be found. The largest columns are those which are formed in very slowly cooling masses. The columnar structure is exhibited by all kinds of lava, and indeed in other rock-masses which have been heated by contact with igneous masses and gradually cooled. The rocks which display the structure in greatest perfection, however, are the basalts. Mr. Scrope first called attention to the fact that the upper and lower portions of lava-streams sometimes cool in very different ways, and hence produce columns of dissimilar character. The lower portion of the mass parts with its heat very slowly, by conduction to the underlying rocks, while the upper portions radiate heat more irregularly into the surrounding atmosphere. Hence we often find the lower portions of thick lava-streams to be formed of stout, vertical columns of great regularity; while the upper part is made up of smaller and less regular columns, as shown in fig. 28. [Illustration: Fig. 28.--Section of a Lava-stream exposed on the side of the river Ardèche, in the south-west of France.] The remarkable grotto known as Fingal's Cave in the Island of Staffa has been formed in the midst of a lava-stream such as we have been describing; the thick vertical columns, which rise from beneath the level of the sea, are divided by joints and have been broken away by the action of the sea; in this way a great cavern has been produced, the sides of which are formed by vertical columns, while the roof is made up of smaller and interlacing ones. The whole structure bears some resemblance to a Gothic cathedral; the sea finding access to its floor of broken columns, and permitting the entrance of a boat during fine weather. Similar, though perhaps less striking, structures are found in many other parts of the globe wherever basaltic and other lava-streams exhibit the remarkable columnar structure as the result of their slow cooling. Portions of basaltic columns are often employed for posts by the road-sides, as in Central Germany and Bohemia, or for paving stones, as in Pompeii and at the Monte Albano near Rome. [Illustration: Fig. 29.--Portion of a Basaltic Column from the Giant's Causeway, exhibiting both the ball-and-socket and the tenon-and-mortise structures.] [Sidenote: OTHER JOINT-STRUCTURES IN LAVAS.]