452 G. E. T'dley — Metanioriphisin of 



(a) The Sleaforcl Bay Section. 



The dolomites of the Sleaforcl Bay area are limited to a com- 

 paratively small exposure on the shore-line. Here they are associated 

 with schists and gneisses intruded by pegmatites and granites in 

 sill-like fashion. 



As exposed on the beach, the marble outcrop is bounded on the 

 east by a sill of hornblende granite and on the west by a coarse 

 pegmatite. At its border it is replaced by ironstone. The exact 

 thickness cannot be made out owing to the poorness of exposures, 

 and large blocks evidently not in situ are present on the beach near 

 at hand. When fresh it is a compact, cream-coloured rock, but on 

 weathering its composite character is indicated by the selective 

 weathering, the calcite, dolomite, and new-formed silicates being 

 attacked differentially, leading to hollows and projections, the latter 

 consisting mainly of dolomite with unaltered silicates. Thin sections 

 of these rocks when examined under the microscope show an 

 interesting suite of minerals, which include calcite, dolomite, 

 forsterite, and an amphibole of the edenite-pargasite group. Earely 

 phlogopite and diopside may be seen. 



Dealing with these minerals we have : — 



(i) Calcite. — Abundantly present in rounded grains up to 1'5 mm. 

 in size. Cleavages are usually present and twinning lamellae (on the 

 j)lane 0112) are rarely to be observed. In sections which show the 

 rhombohedral cleavages intersecting at the cleavage angle of 

 74 degrees the twinning lamellae are seen to intersect the acute angle 

 between the cleavages, and are parallel to the longer diagonal of the 

 rhombohedron. 



The distinction between calcite and dolomite was primarily 

 effected by using Lemberg's solutions. Uncovered sections were 

 treated with a dilute solution of ferric chloride for two minutes, 

 washed and then immersed in ammonium sulphide solution, washed 

 and finally treated with potassium ferricyanide, by which the 

 precipitated ferrous sulphide is converted into Turnbull's blue. 

 By this means calcite is stained blue, whilst dolomite remains 

 unstained. This method has been found very satisfactory in 

 differentiating these carbonates. The calcite grains in these rocks 

 are remarkable for their turbidity and the dolomite for its limpidity. 

 The clouded character is due to minute inclusions, some of which 

 are transparent and of high refractive index. They are often quite 

 ragged in outline, and appear not to be disposed along any particular 

 direction. Others, usually more minute, are opaque, and their 

 nature is not clear. In some calcite grains there are small granules 

 of dolomite, clear and unstained, suggestive of a ramifying inter- 

 growth. In one slice (115a) a primary twin of calcite-twin plane — 

 (lOll) — is shown, the two parts having their vertical axes inclined 

 at approximately 90 degrees, and the twin being bounded by prism 

 and rhombohedron faces. 



