Prof. G. A. J. Cole — Meshivork-stnwtures in Rock Sections. 253 



of muscovite, which I recently received for examination from the 

 Geological Survey of the United Kingdom, The cross-sections of 

 the mica, coloured in pinks and greens between crossed nicols, stand 

 out in two distinct series perpendicular to one another, so as to 

 suggest a remarkable double foliation. On rotation of the stage, 

 however, all chance of misconception was removed. 



It would be unwise to suggest that this or that rectangular 

 mesh, described by various authors, may be a structure visible only 

 between crossed nicols, and with no existence in reality. But it is 

 easy to extend the observation to the mesh-structure of decomposing 

 olivine, and to dispose of some of the difficulties as to the presence 

 or absence of rectangularly arranged fibres in particular examples. 

 I am indebted to Miss C. A. Eaisin, B.Sc, for the loan of several 

 sections of serpentine from the Eauenthal, for comparison with 

 other serpentines in my own collection ; and the former have 

 proved of special service. While fully agreeing with Miss Eaisin ^ 

 and Professor Bonney ^ as to the former existence of olivine in the 

 Eauenthal rock, I would not lay great stress upon the rectangular 

 appearances seen in the meshwork of this or other serpentines.^ 

 Where the mesh-structure is on a coarser scale, or on using a higher 

 power, rectangular mesh -structures are rarely seen, since the eye 

 does not readily take in the requisite number of illuminated bars 

 at the same time ; where, however, the structure is finer — i.e., where 

 the original cracks of the olivine were closer together — a rectangular 

 effect is readily produced, by the prominence of the doubly re- 

 fracting decomposition-products lying along those cracks or portions 

 of cracks that happen to be perpendicular to one another. 



I cannot help thinking that von Drasche's* " deutliches quad- 

 ratisches Netzwerk " in some parts of the "serpentine-like rock" 

 of Windisch-Matrey may have been quite local, or even due to 

 accidents of observation, especially as the author states that the 

 fibres in most parts of the rock intersect at very various angles. 

 The rectangular effect is well shown in his plate, fig. 2 ; and 

 prominence was given to a similar arrangement by Hussak^ during 

 his investigation of the antigorite-serpentine of Sprechenstein, near 

 Sterzing in Tyrol. This rock was photographed by Cohen ^ in 

 the same year, from one of Hussak's specimens, between crossed 

 nicols, and the figure is still better known from its occurrence in 

 Eosenbusch's " Massige Gesteine." In his description of the plate, 

 Eosenbusch iises for the arrangement the term " Balkenstructur," 

 which is adopted also by Zirkel.' Oathrein ^ also comments on 



^ "The Nature and Origin of the Eauenthal Serpentine": Quart. Journ. Geol. 

 Soc, vol. liii (1897), p. 255. 



- "Note on Specimens of the Eauenthal Serpentine": Geol. Mag., 1887, p. 68. 



•^ See Miss Eaisin, op. cit., p. 263. 



* "Ueber Serpentine und serpentinahnliche Gesteine," Tscherm. Mitth., 1871, p. 5 

 (Jahrb. d. kk. geol. Eeichsanstalt, Bd. xxi). 



5 "Feber einige alpine Serpentine," Tscherm. Mitth., 1883, p. 70. 



^ " Sammlung von Miki'ophotographien," pi. Ixv. 



■^ "Lehrbuch d. PetrograiDhie, " 2te. Auflage, Bd. iii (1894), p. 384. 



8 "Beitrage zur Petrogr. Tirols" : Neues Jahrb. fiir Min., 1887, Bd. i, p. 152. 



