138 Correspondence — Mr. J. J. H. Teall. 



Now I find in all sections of the Rill serpentine that I have 

 examined irregular grains of a colourless mineral having the refrac- 

 tive index of felspar, so far as one can judge of this by the relief of 

 the section and the character of its surface. This mineral always 

 polarizes in the neutral tints of the first order in sections in which 

 olivine and augite would give as a rule chromatic polarization. It 

 frequently shows, moreover, a fine lamellar twinning, and 

 sometimes two sets of parallel lamellge may be seen inter- 

 secting each other at a high angle. It has been rendered turbid 

 in. places by granular decomposition products. Now I know of no 

 mineral except felspar which possesses all these characters. Professor 

 Bonney vaguely suggests that it is augite or diallage. I am, of 

 course, aware that augite does show multiple twinning ; but I cannot 

 possibly regard this mineral as augite. In one case in which the 

 extinction of the two sets of twin lamellae were approximately 

 symmetrical with reference to the trace of the face of composition, 

 the combined angle was 53°. Now, if the crystal were augite, 

 twinned according to the ordinary law, such a section could not 

 possibly be cut approximately at right angles to an optic axis ; ^ and, 

 therefore, in slides of the thickness used, could not possibly polarize 

 in the neutral tints of the first order, as it actually does. 



Another very important point is the existence of two sets of 

 lamellas intersecting at a high angle. This is perfectly easy to 

 understand on the assumption that the mineral is felspar; but, so 

 far as I know, inexplicable on the assumption that it is augite. 



To sum up. As the mineral possesses the refractive power of 

 felspar, the double-refraction of felspar, the twinning of felspar, and 

 the mode of alteration of felspar, so far as we are able to judge of 

 these characters under the microscope, I adhere most firmly to my 

 original statement. 



Section of felspar in the Eill serpentine, showing- cross-twinning ; Nicols crossed. 

 Magnified 80 diameters. 



In my remarks on the Eauenthal serpentine I have simply 

 followed Weigand, and I must leave him to take care of himself, 

 as, no doubt, he is well able to do. I may remark, however, that 

 the main point of Weigand's paper, so far as it relates to the 

 Eauenthal rock, is to prove that serpentine has been largely pro- 

 duced by the alteration of hornblende, and that the serpentine so 

 formed can be distinguished from that produced by the alteration of 

 olivine. The slides of specimens purchased from Sturtz amply 



' On referring to Fouque and Levy (Min. Micrographique, p. 355), it will be seen 

 that the section in question, if of augite, would be out of the zone 100 : 010 and would 

 make an angle of about 35° with the ortho-pinacoid. 



