68 Prof. T. G. Bonney — On the Rauenthal Serpentine. 



tributed. The one — almost colourless — clearly is or has been a horn- 

 blende, as described by Herr Weigand. It shows, even when con- 

 verted into serpentine, the characteristic cleavages parallel to oo p in 

 cross-sections and the parallel lines formed by these in longitu- 

 dinal sections. Occasionally portions of the mineral still remain 

 unaltered. The other constituent is a pale sap green in colour; it 

 consists of a rather irregular network of a doubly refracting mineral, 

 with interspaces chiefly occupied by an isotropic mineral. This 

 meshwork can be readily seen by transmitted light, but is rendered 

 more distinct by using crossed Nicols, when the ' strings ' appear 

 either Avhite or pale golden yellow. This structure, and this mineral 

 or association of minerals are perfectly familiar to me as the result of 

 the alteration of olivine ; there are various minute structural dif- 

 ferences between the isotropic parts of this and the most highly 

 altered portions of the other mineral, which it is almost impossible 

 to describe without entering into details which would be tedious, but 

 which become readily apparent to a practised eye after a little study, 

 so that while I will not undertake to say that I can distinguish every 

 grain of the two constituents, I have no hesitation as to distinguish- 

 ing them in at least five-sixths of the rock. Hence I cannot admit 

 the general accuracy of my friend's statement, founded upon Herr 

 Weigand's paper : " Microscopic examination shows that the rock 

 differs markedly from normal olivine serpentine. It consists of a 

 transparent green or colourless substance,' in which the irregular 

 ' maschenstructur ' due to the separation of the iron oxide along the 

 original cracks of olivine is absent." But the "maschenstructur" 

 of a serpentine derived from olivine is not due " to the separation of 

 the iron oxide," but " to the original cracks " {i.e. the imperfect 

 cleavage planes) of the mineral itself. It is, indeed, very commonly 

 exhibited in great perfection by the deposition of iron oxide, which 

 plays the part of an injection in an organic structure ; but it is quite 

 a mistake to suppose that the iron oxide is always separated and 

 deposited in this way,^ The ' mesh structure ' in the Kauenthal rock is 

 not seldom divided down the middle by a line (indicating the original 

 cleavage plane) ; this is sometimes made more distinct by a slight 

 deposit of iron oxide, and the minuter structures of these strings- 

 agree perfectly with those in a normal serpentine. Thus the above- 

 quoted sentence would be more correctly worded as follows : — " It 

 consists of a transparent green and a colourless substance," and the- 

 remainder of the sentence would then appl}'' to the latter of these, 

 viz. the altered hornblende. As to the relative amounts of the two 

 minerals, it is difficult to speak exactly, as they are rather irregularly 

 mixed ; but in no case, I think, does the hornblendio constituent 

 dominate in the field of view under a one-inch objective, and on the 



' I observe that very commonly there is a slight green band between the cracks 

 (cleavages) of this mineral, as though a liquid had been injected from without. I 

 have noted the same thing in the case of the pyroseuic minei'als present in other ser- 

 pentines. The mineral may be derived from the olivine and " injected " during the 

 hydration of the olivine, which process must set up considerable strains and pressures. 



^ See, for instance, Wadsworth's Lithological Studies, pi. iv. fig. 4. The decided 

 green colour is probably due to imperfect separation of the iron. 



