128 DK. A. L. DU TO IT ON THE [vol. lxxv, 



Outside the phlogopite-calcite zone comes the shell of ophi- 

 calcite, a distinctly banded rock composed of calcite and greenish- 

 yellow serpentine very much like an ' Eozoon Marble ' ; this in 

 turn fades away into the normal white, crystalline, dolomitic 

 marble. 



Before discussing the chemical changes involved, it would be 

 well for comparison to consider several other marginal trans- 

 formations of a more or less similar type. 



Slide 3387, cut from a granitic dyke on the western side of the 

 farm Bavaria, shows quartz, microcline, orthoclase, albite-oligoclase, 

 and sparsely scattered p}'roxene — the latter yellowish in colour 

 with marked dispersion, and bordered by deep green pleochroic 

 aegirite — while sphene is abundant. V. M. Goldschmidt 1 has, it 

 might be pointed out, similarly observed an aegirite shell sur- 

 rounding the pyroxene (hedenbergite) in contact-altered limestone 

 in the Christiania region. Tiny patches of calcite are scattered 

 about the slide, and obviously arise from undigested grains of 

 marble. 



A dyke on the western side of La Joncquet, close to the main 

 contact, proves from the section (3377) to be a banded micro- 

 cline-granite with a little augite and pale hornblende — the latter 

 being generally intergrown with the pyroxene in parallel position, — 

 biotite, sphene, and some graphite. This intrusion, as would be 

 anticipated, approximates closely to the parent gneisses of the 

 area, which, as stated, carry biotite and frequently hornblende, 

 but not usually pyroxene. 



The small granite-dykes and veins are almost invariably bordered 

 by a zone not exceeding a few inches in thickness of pale 3 r ellowish 

 phlogopite occurring in Hakes set in planes roughly normal to the 

 contact ; and this reaction-zone is particularly valuable when we 

 are considering certain lumps of granite isolated in the marble 

 that might, when not irregular in outline, be readily taken for 

 pebbles or boulders, in view of the known instances of erratics in 

 the Chalk or Carboniferous Limestone, such as have been cited bv 

 Hatch & Kastall. 



From other areas of crystalline marbles invaded by plutonic 

 rocks the phenomenon of the disruption of dykes immediately 

 following their injection, whereby portions of the intrusions have 

 been nipped off: and come to be isolated in the marble, is well 

 known, while recrystallization of the country-rock destroys all 

 effects of shearing in the mass. All the stages in such disruption 

 can be found by search in the Marble Delta, and it is noteworthy 

 that the broken and separated portions of granite are commonly 

 still partly (or even wholly) surrounded b}^ phlogopite reaction- 

 shells. One interesting case is that of a vein about 2 cm. broad, 

 where there is a development of pyroxene, not only on the flanks 

 of the broken parts, but even along the irregular surfaces of 



1 Skrifter Videnskapsselsk. Kristiania, Math.-Nat. Klasse, vol. i (1911) 

 pp. 342-44. 



