254 Prof. G. A. J. Cole — Meshicorli-stnictures in Rock Sections. 



a " Balkennetz," and agrees with Hussak on its significance in 

 indicating the pyroxenic origin of certain serpentines. F. Becke/ 

 as is now well known, has diminished the importance of Hussak's 

 suggestion by indicating similar structures in an antigorite-serpentine 

 derived from a true olivine-rock ; and he regards the antigorite 

 plates as developed along the cleavages of the original olivine 

 grains. This is the view taken by Miss Raisin in dealing with the 

 rectangular structures of serpentine ; while Professor Bonney, with 

 much probability, regards the irregularity of the mesh-structure 

 ordinarily seen as due to the imperfect character of the cleavages 

 in olivine. In 1891 I wrote ^: "These needles, picked out by the 

 use of the polariscope, are so frequently at or nearly at right angles 

 to one another as to suggest their development along the cleavage- 

 planes of the mineral that has been pseudomoriDhosed. It is often 

 stated that the serpentine in such cases has been derived from 

 pyroxene ; but the structure is extremely common in company with 

 others referred as certainly to olivine." Becke's paper on the 

 Stubachthal has fully confirmed the latter statement ; but my present 

 note is intended to suggest that a true rectangular meshwork, con- 

 tinuous thi'oughout the whole of an olivine grain, is not so frequent 

 an occurrence as might at first appear. In the first place, the more 

 brightly illuminated part of the cross-section of a curving " anti- 

 gorite " or other plate may catch the eye and give a fictitious 

 appearance of regularity. In the second place, a second rectangular 

 meshwork may sometimes appear during rotation within the grain 

 that is being observed, the former one being now extinguished ; 

 and we thus see that several systems of cracks, with corresponding 

 alteration-films, occur, and that these cracks are picked out in pairs 

 by the use of polarised light. No one can deny that olivine is often 

 broken up into approximately rectangular blocks, even in the fresh 

 state ; for the larger crystals observable in many basalts show this 

 character when examined with the lens. Consequently, a rudely 

 rectangular structure is common in the products of alteration. But 

 I believe that its regularity is easily liable to be exaggerated ; and 

 the same consideration becomes far more important when extended 

 , to sections of metamorphic rocks, containing tufted chlorites, micas, 

 and so forth, lying in all directions through the mass. 



In conclusion, the reality of the optical effect above described 

 cannot be more happily demonstrated than in the photographs 

 accompanying Mr. Arnold-Bemrose's paper^ " On a Quartz-Rock in 

 the Carboniferous Limestone of Derbyshire." The irregularly-lying 

 quartz needles have been photographed in section between crossed 

 nicols ; and a rectangular effect, which is just apparent in fig. 2, 

 becomes markedly so in the more fine-grained material shown in fig. 3. 



1 " Olivinfels und Antigorit-Serpentin aus dem Stubachthal," Tscherm. Mitth., 

 Ed. xiv (1895), p. 275. 



2 "Aids in Practical Geology," p. 164. The objects here called "needles" are 

 the cross -sections of the little plates developed along the cracks. 



3 Quart. Journ. Geol. Soc, vol. liv (1898), pi. xii. 



