252 Prof. G. A. J. Cole — Meshwork-structures in Roch Sections. 



11. — On Meshwork- structures observable in Microscopic 

 Sections of Eocks. 



By Grenville A. J. Cole, M.R.I.A., F.G.S., 

 Professor of Geology in the Royal College of Science for Ireland. 



THIS note deals with a very simple matter, which, is perhaps 

 familiar to many workers with the microscope. I believe, 

 however, that it is not referred to in ordinary text-books, and a word 

 or two in reference to it may be useful. 



From time to time, meshwork-structures have been described, 

 either in the felted ground masses of igneous rocks or in the pro- 

 ducts of the decay of ferromagnesian minerals ; and it is frequently 

 remarked that a rectangular ai-rangement of the constituents of the 

 mesh is clearly brought out when the section is examined between 

 crossed nicols. 



One of the most beautiful experiments to illustrate the separation 

 of materials in a crystalline form from a state of fusion is to receive 

 a drop of " candle-grease " from an ordinary commercial candle 

 upon a microscopic slip. Place a circular cover-glass upon it, heat 

 until the material is well fused, and lay it beneath a power 

 magnifying some 100 diameters and between crossed nicols. If 

 the instrument has been previously focussed for a glass slip of the 

 right thickness, no stage of the process of crystallisation will 

 be lost. At first, as cooling proceeds, tiny specks emerge from 

 the dark ground, like stars in an evolving universe. Gradually 

 a delicate meshwork of fine and somewhat wavy rods spreads 

 inwards from the cooling outer circle of the cover-glass. These 

 rods rapidly thicken, while their interstices finally become filled 

 up. But the meshwork effect is still perceptible, and a rectangular 

 arrangement of the crossing fibres is strikingly apparent. 



On I'epetition of the experiment, it may appear surprising that 

 the rectangular mesh extends regularly across the slide, as if some 

 polarity existed between the minute crosses of which it is com- 

 posed. On still further repetition, it will be plain that the two 

 series of fibres always lie at 45° to the diagonals of the crossed nicols. 



On rotating the stage, the meshwork does not rotate ; that is, new 

 fibres come into the positions of the former ones, and therefore into 

 the same positions of prominence between crossed nicols. 



The secret is, of course, that the crystals of stearic acid are in 

 these positions in a state of maximum illumination, while those 

 lying in other positions have a much feebler effect upon the eye, or 

 may even be totally extinguished. Even if the crystals of such an 

 aggregate have oblique extinction, in every radiating or bap-hazard 

 group there are likely to be four crystals which are at 45° to their 

 positions of extinction ; these will impress themselves upon the eye, 

 and a cross will be the result, though its components will not have 

 their longer axes at 45° to the diagonals of the nicols. The efiect 

 is far more noticeable where, owing to the thickness of the section, 

 or to the strength of the double refraction of the material, the 

 interference-colours are of a high order. I have seen no more 

 striking example than in a schist with irregularly scattered plates 



