F. Chapman — Artificial Perlitic Structures. 79 



is clear that we shall have to regard all limestones as of compound 

 origin, partly organic, partly detrital, partly as chemical precipitates 

 — the proportion of each to the other varying greatly to a much 

 greater extent than has yet been recognized. 



VII. — On a Method of Producing Perlitic and Pumiceous 

 Structures in Canada Balsam. 

 By Frederick Chapman. 

 rpHE interesting phenomenon exhibited in some glassy rocks, such 

 JL 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. 1 



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. 



For a cabinet specimen one cannot do better than to take a 

 disc of glass of about 1J inches in diameter; or if it is wanted 

 for microscopic examination it may be necessary to use a 3 x 1 

 slip. One surface of the glass is then ground on emery powder ; 

 meanwhile Canada Balsam is prepared in an evaporating basin by 

 heating to brittleness. The balsam is taken whilst beginning to 

 cool and poured on the glass, which has been slightly warmed to 

 allow the balsam to flow without including bubbles. When the 

 balsam has cooled slowly to a firm condition, but not quite cold, 

 the plate is plunged into a vessel of cold water. Immediately this 

 is done the rectilinear and subsequently the curved cracks appear 

 throughout the entire layer of balsam. With a little care, very 

 perfect and beautiful examples of the perlitic structure may be 

 prepared in this way. 



1 On the Artificial Production of Perlitic Structure, Geol. Mag. Dec. II. 

 Vol. VII. 1880, p. 115. 



