The interesting phenomenon exhibited in some glassy rocks, such as obsidians and pitchstones, known as the perlitic structure, consists of a series of rectilinear and curved cracks. It has been produced artificially in Canada Balsam, and described by Mr. Grenville A. J. Cole, F.G.S.

Hitherto, any attempts of mine to produce the curved or secondary cracks on a glass plate were unsuccessful (the only result being primary cracks and appearances of air films between the glass and balsam); until noticing their occurrence on a rough mounting plate, it became evident that a roughened glass surface was the requisite thing in order to secure perfect cohesion. It is probable, therefore, that the specimen which Mr. Cole produced occurred on a ground portion of the glass plate.