﻿Vol. 6 1.] MICROSCOPIC STRUCTURE OF SERPENTINE. 695 



us to identify the mineral. Where it originally contained a fairly- 

 high percentage of iron, opacite (sometimes as belonites) is de- 

 posited more or less interruptedly along these planes, and thus 

 indicates their position, where but for this record that would have 

 been indistinguishable; but, when the percentage is low, we find only 

 a few chance black specks or none at all. Even then, however, the 

 mineral, instead of being practically colourless, sometimes has a 

 pale-green tint (indicating no doubt a small amount of iron). It 

 may then show a slight pleochroism, pale green with vibrations 

 parallel with the cleavage-planes, and pale buff perpendicular to 

 them. The polarization-tints of serpentinized enstatite are generally 

 the dull bluish- whites of the first order, rising occasionally to the 

 pale yellow ; grains cut nearly parallel to the cleavage-planes 

 present a slight, but peculiar, wavy aspect, and with crossed nicols 

 show glimmering patches of dull bluish-white on a dark ground — 

 the result, no doubt, of some curvature in the structure. 



Occasionally, as observed by Carvill Lewis in the diamantiferous 

 rock of South Africa, 1 the enstatite (which contains 34*91 per cent, 

 of magnesia and 4*99 of ferrous oxide, thus lying, according to 

 Tschermak, on the border-line between enstatite and bronzite) alters 

 into a fibrous bastite, one variety of which is a dark indigo-blue. 

 In my slices of the diamantiferous rock the occurrence of this variety 

 is doubtful, but it appears in a saxonite 2 (boulder) where the enstatite 

 has a well-defined pinacoidal cleavage, but hardly any other signs 

 of change. 3 At the same place a fragment of a coarse enstatite- 

 rock was also found. 4 



In conclusion, we may mention one or two rather exceptional 

 forms of change. In the altered bastite the fibrous structure 

 may become more pronounced, or the mineral assume a slightly- 

 pleochroic pale-brown instead of a green tint ; or two species of it, 

 a ferriferous and a non- ferriferous, may be present; or, very rarely, 

 portions in a less advanced state may remain as small, slightly- 

 interrupted prismatic rods (defined, of course, by the pinacoidal 

 cleavage); or (as occurs in a dyke consisting practically of enstatite 

 from south of Penrhyn Fadog, Anglesey 5 ) the grains, slightly 

 pleochroic, show polarization-tints ranging from the bluish-white 

 to the pale yellow of the first order, are apt to be bent, and break 

 up at the ends into a rather fibrous mass, of serpentinous aspect, 

 which acts more feebly on polarized light, sometimes remaining 

 dark between crossed nicols. 



A poikilitic enclosure of olivine, so far as our experience goes, is 

 rare, unless the crystals exceed the ordinary size (that is, about a 

 third of an inch in diameter). At the Lizard it is practically 

 absent, one or two slices only showing traces of it on a small scale, 



1 ' The Genesis & Matrix of the Diamond ' 1807, pp. 19-21. 



2 Saxonite of Wadsworth, or harzburgite of Rosenbusch ; that is, oli vine- 

 en statite rock. 



3 I am doubtful whether the colour indicates more than a local staining due 

 to the accidental presence of some mineral. [T. G. B.] 



4 See Proc. Roy. Soc. vol. lxv (1899) p. 231. 



5 Coll. C. A. R., No. 661. 



