A PERLITIC PITCHSTONE FROM TWEED RIVER. 313 



and is probably quite analogous if we remember that one is pro- 

 duced in a thin layer of substance and the other in the interior of 

 a solid mass. In fact we may consider the artificial structure as 

 a two-dimensional phase of the natural structure. Certain points 

 of difference follow from this, to which I will allude later on. 



As the method of producing the structure in canada balsam 

 does not appear to be very generally known, I may perhaps be 

 permitted to refer to one or two points in connexion with it. If 

 a layer of balsam be spread on a polished slip of glass (or a cover 

 glass) and heated until it becomes distinctly yellow in colour and 

 then plunged into cold water, the balsam will be found to be 

 traversed by numerous fine cracks which may be described as 

 polygonal. In this case perlitic cracks are not developed, or only 

 to a very slight extent. If however, the surface of the glass slip 

 be first uniformly ground and the process repeated, the balsam 

 will be found to exhibit, in addition to the polygonal cracks, very 

 beautiful perlitic structure — the perlites occupying the interspaces 

 between the polygonal cracks. If, instead of plunging the slip 

 into water it be laid on a piece of glass the cracking takes place 

 more slowly, and may be watched under the microscope. In this 

 case the polygonal cracks make their appearance first, and then 

 each individual perlite springs suddenly into existence, producing 

 the impression of a tremor in the little mass of balsam. The 

 higher the balsam is heated and the more quickly it is cooled the 

 smaller is the interval of time between the two sets of cracks, 

 until it becomes imperceptible to the eye. 



In the structure so produced we may notice the following points 

 (Plate 45, fig. 4.) 



First, that the perlites always occur in the interspaces of a set 

 of polygonal cracks and are formed subsequently to the latter, 

 though the interval of time may be very small. 



Second, each perlite is formed of segments of one or more 

 spirals like curves — the radius of curvature tending to diminish 

 as the curve is followed round. (Allowance must be made for 



