part 2 THE GEOLOGY or THE MARBLE DELTA. 127 



intergrowth of colourless albite, thus forming perthite ; the pyro- 

 xene is a rich green pleochroic soda-augite in irregular areas, and 

 often shows alteration along cracks and at the margins — especially 

 where in contact with the quartz — to a dull bluish mass seen 

 under a high power to consist of a fibrous intensely-pleochroic 

 soda -amnhi bole ; there is a good deal of sphene and a little apatite. 

 (See P1*IX, tig. 1.) 



The coarser rock nearer the interior carries a pyroxene that is 

 not dark blue-black but greyish-green macroscopically, colourless 

 and non-pleochroic in thin section ; it is, therefore, not a soda- 

 augite. At its extreme margin the granite is devoid of quartz. 

 but carries some albite and little patches of calcite. 



The reaction-zones around the deeply-embayed granite are three 

 in number, namely, a diopside-scapolite rock, a phlogopite-calcite 

 rock, and a zone of ophicalcite, the arrangement hence differing 

 a imewhat from that in the threefold aureole described by Hatch 

 & Kastall, which, inferentially, must have belonged to a more or 

 less isolated protuberance from the sill, since quarried away. 



The innermost zone is a pale, faintly greenish to bluish, pris- 

 matic aggregate, the individual crystals in which are set very 

 generally with their long axes approximately normal to the granite- 

 contact. Thin sections (3383, 3412, 3413) show that the bulk of 

 it is formed of colourless diopside ; scapolite, fresh in a few places, 

 has mostly been altered to a brownish fibrous material, probably a 

 zeolite: it tends towards idiomorphism. There is some calcite, an 

 almost colourless phlogopite, and a good deal of water-clear felspar 

 with faint repeated twinning ; the felspar was the last mineral to 

 form, and is for the greater part albite or albite-oligoclase. 



The rock described by Hatch Sc Kastall (under the heading 3 a), 

 from a lump picked up in this quarry, is identical with the above 

 and cannot he other than a fragment from this zone. 



Towards the outer border of the diopside-scapolite zone phlogo- 

 pite appears, at first in isolated crystals often exceeding 1 cm. in 

 diameter, usually split up by thin 61ms of silicates between the 

 folia, but with faces other than the basal pinacoids present as well; 

 calcite becomes abundant next, and is rather coarsely crystallized. 

 The dark mica -olivine -spinel zone (3 b) observed by Hatch A: 

 Kastall was not noticed in the section covered by fig. 3, but 

 from among the spoil in the quarry fragments were picked up 

 possessing very similar characters. 



Slide 3111, for example, showed abundant forsterite (partly 

 altered to serpentine without the separation of magnetite), calcite, 

 a very little diopside, and a fair amount of rutile, phlogopite, and 

 c tlourless spinel grey or faintly pink macroscopically. The most 

 peculiar character of the spinel is its generally skeletal outline in 

 thin section, due to the inclusion of the other minerals — principally 

 calcite (PI. IX, fig. 2) : the faces of the octahedron are partly 

 present; but the crystal is sometimes a mere shell, and it therefore 

 differs in habit from that in the specimen described by Hatch & 

 Kastall, in which the mineral occurs in octahedral crystals. 



