Various investigations of the Perlitic structure in igneous rocks have resulted in its being regarded as a product of unequal contraction in a cooling mass of lava. Professor Bonney, in a paper on Columnar, Fissile, and Spheroidal Structure, has compared it to the roughly-concentric joints to which, in certain basalts, the spheroidal character is due; and Mr. S. Allport, in his account of certain Ancient Devitrified Pitchstones and Perlites, writes that “an examination of all the facts leads to the conclusion that the perlitic texture is purely a phenomenon of contraction.” Of this, then, there has practically been no doubt; but, so far as I am aware, the structure has not until recently been imitated by artificial means.