Dr. J. 8. Hyland—Zonal Structure in Olivine. 493 
division between the fresh and decomposed portions was mostly 
parallel to the crystallographic outlines.’ There was, however, 
no break in the optical continuity, such as the presence of zonal 
structure would demand. 
A microscopical examination of some similar rocks from the same 
locality has led to the gratifying discovery of true evidence of zonal 
structure in this mineral. Fig. 1 shows a corroded olivine exhibiting 
this phenomenon. The inner zone is shaded in 
_ order to graphically represent its outline with ug L. 
reference to the contours of the crystal. The 
zones are dissimilar in their optical behaviour, 
the difference between their extinction-angles 
being as much as 6°. In transmitted light there 
is a trace of the structure apparent; the upper 
line of division being a plane of fracture, the 
lower being marked by glass inclusions. 
The literature on the subject is not extensive. 
Van Werveke, in 1879, seems to have been 
the first to suggest the possible occurrence of 
zonal structure in this mineral. In his paper on the Palma basalts 
he mentioned the presence in this constituent of glass inclusions, 
which were arranged parallel to the crystallographic outlines. This 
induced him to expect zonal structure.? In the same year Hofmann 
formed a similar opinion as the result of his observations of the 
presence of “zones which were different in their optical characters 
and in their manner of alteration.” * Bruno Doss, in his paper on 
Syrian basalts, described and figured olivines composed of concentric 
layers, but was able only in one instance to note a variation in the 
polarisation tints. Stock has also mentioned a similar experience.® 
I fail, however, to find any instance recorded where the difference 
in the extinction-angles was so great as to allow of its accurate 
determination. 
Crystals grow by accretion, that is, by the addition of matter to 
their external surfaces. If this addition be regular and constant, 
the crystal formed will be a correct type of the species. But, if the 
growth be intermittent, complications will arise. In this respect a 
crystal is like a living organism: it is affected by its environment. 
The crystal modifies its surroundings, and is in turn modified by them : 
there is action and reaction between it and its environment. Inter- 
mittent growth must, accordingly, tend to produce zoned structure. 
In the case of isomorphous mixtures like augite and felspar this 
1 “Ueber die Gesteine des Kilimandscharo und dessen Umgebung.’’ Tschermak’s 
Mittheilungen, 1888, vol. x. pp. 224 and 226; also plate vii. fig. 2. 
2 The mineral gives reactions characteristic for olivine. 
3 « Beitrag zur Kenntniss der Gest. d. Insel Palma.’’ Neues Jahrbuch fur 
Mineralogie, etc., 1879, p. 820. 
4 «Die Basaltgest. des stidlichen Bakony,’’ Budapest, 1879, pp. 27 and 198. 
> <¢Die basaltischen Laven und Tuffe der Proyinz Hauran, etc.,’’ ‘I'schermak’s 
Mitt. 1886, vii. p. 488; plate ix, fig. 32. 
6 «Die Basaltgest. des Lobauer Berges,’’ Tschermak’s Mitt. 1888, ix. p. 487; and 
plate ix. fig. 2. 
