Occasionally basaltic lava-streams exhibit other curious structures in addition to the columnar. Thus some basaltic columns are found divided into regular joints by equidistant, curved surfaces, the joints thus fitting into one another by a kind of ball-and-socket arrangement. Sometimes we find processes projecting from the angles of the curved joint-surfaces, which cause the blocks to fit together as with a tenon and mortise. This kind of structure is admirably displayed at the Giant's Causeway, Co. Antrim, in the North of Ireland. A portion of a basaltic column from this locality is represented in fig. 29. [Illustration: Fig. 30.--Vein of green Pitchstone, at Chiaja di Luna in the Island of Ponza, breaking up into regular columns, and into spherical masses with a concentric series of joints.] While the ordinary columnar structures are very common in basalts, the ball-and-socket and tenon-and-mortise structures are exceedingly rare. The question of the mode of origin of these remarkable structures has given rise to much discussion, and the opinions of geologists and physicists are by no means unanimous upon the subject. Sometimes we find masses of lava traversed by curved joints, and occasionally we find curious combinations of curved and plane joints, giving rise to appearances scarcely less remarkable than those presented by the columns of the Giant's Causeway. Some of the more striking examples of this kind have been described and explained by Professor Bonney. [Illustration: Fig. 31.--Illustration of the 'Perlitic structure' in glassy Rocks. a. Perlltic structure, as seen in a lava from Hungary. b. The same structure, artificially produced in Canada Balsam during cooling. ] [Sidenote: PERLITIC-STRUCTURE IN LAVAS.] In the Ponza Islands there occurs a remarkable example of a columnar pitchstone, which is also traversed by a member of curved concentric joints, causing the rock to break up into pieces like the coats of an onion. This remarkable rock-mass is represented in fig. 30. A very similar structure is often seen in certain glassy lavas, when they are examined in thin sections under the microscope. Such glassy lavas exhibit the peculiar lustre of mother-of-pearly doubtless in consequence of the interference of light along the cracks. Lavas exhibiting this character are known to geologists as 'perlites.' The perlitic structure has been produced artificially by Mr. Grenville Cole in Canada Balsam, and by MM. Fonqué and Michel Lévy, in chemically deposited silica. See fig. 31.