314 W. F. SMEETH. 



distortion according to the shape of the polygonal interspaces). 

 This spiral character can be clearly seen, and as a consequence 

 the curves are not closed. Occasionally a curve may divide 

 tangentially into two, or two may join into one, due doubtless to 

 want of homogeneity or to a local disturbance. 



Before applying these observations to the perlitic structure of 

 a mass of rock, it will be necessary to try and form some con- 

 ception of what I would call, the ideal perlitic surface. We have 

 an approach to one section of this surface shown in the layer of 

 balsam as a spiral curve, and we may take the external shape of 

 a perlite to be spheroidal and that the outer surface completely 

 encloses all the inner turns. It is evidently not possible to have 

 a surface every section of which will be a spiral, and we are forced 

 to assume that the surface is rolled spirally on an axis, each turn 

 enclosing the previous ones, and all the turns touching each other 

 tangentially where the axis intersects them. We have approxi- 

 mations to such a surface in the foraminifera nummulites and 

 fusulina, the former being generally compressed and the latter 

 drawn out along the axis. 



A section of such a surface at right angles to the axis will give 

 a spiral (Plate 46, fig. 3). A section passing through the axis will 

 give a figure something like fig. 4, all the curves touching at two 

 points on the axis. When the section passes through only one of 

 these points we get fig. 5, and when it does not cut the axis 

 between these points we get curves like fig. 6. 



So far I have assumed this surface to be a continuous one. But 

 even in the canada balsam the spirals are only segments, and 

 therefore represent portions of a surface or surfaces similar to the 

 ideal one. Again, in the balsam the curves all represent sections 

 of the surface at right angles to the axis consequently the balsam 

 may be considered as a section through a number of perlites whose 

 axes are all parallel to each other and at right angles to the section. 

 To such a group of perlites I may perhaps be permitted to apply 

 the term ' polarized perlites,' and this arrangement is undoubtedly 

 due to the mode of production on a plane surface of ground glass. 



