ART. 2. PETROLOGY AT GOOSE CEEEK SHANNON. 17 



and is easily distinguished from the other by the fact that the feld- 

 spar, all of which has a refractive index well below that of the bal- 

 sam, is muddy from the presence of minute dustlike inclusions, 

 which are too small to be determined. 



Apatite occurs sparingly as small crj'^stals, usually disseminated in 

 the micropegmatite. 



Iron ore forms large triangular skeletal individuals, probably late 

 replacements, which are developed regardless of the other minerals, 

 one such spongy mass looping around feldspars, pyroxene, and mi- 

 cropegmatite indiscriminately as shown in the photomicrogragh (pi. 

 5, lower) . The triangular skeletons, which average 5 mm. in diame- 

 ter, are made up of plates parallel to the faces of the octahedron 

 which indicates that the mineral is isometric magnetite rather than 

 trigonal ilmenite, a fact further established by the fact that they are 

 lifted by a hand magnet. Material of a large skeleton crystal ground 

 and purified with a horseshoe magnet gave, however, a strong reac- 

 tion for titanium by the hydrogen peroxide test, so that the mineral 

 is titaniferous magnetite. 



Biotite was noted in thin sections of the rock only as small flakes 

 which are distributed around the iron ore at its contact with feldspar, 

 and seems definitely to be a reaction product. Numerous occurrences 

 of secondary biotite formed by reaction between iron ore and feld- 

 spar have been cited by Sederholm'' and this seems to be a typical 

 case. A similar relationship has been noted for a considerable part 

 of the biotite in the diabase of normal grain. This secondary biotite 

 of the diabase pegmatite is pleochroic in pale brown and dark green- 

 ish broAvn, the direction of maximum absorption being, as is usual for 

 biotite, parallel to the basal cleavage in which it differs from the 

 biotite seen in the normal fine grained diabase, the direction of max- 

 imum absorption in which was anomalously perpendicular to the basal 

 cleavage. Although not seen in any of the thin sections there are 

 nests of coarse black biotite flakes occasionally in evidence in hand 

 specimens, which are probably original consituents of the rock. One 

 such nest was 5 mm. across and was made up of flakes 2 mm. in diam- 

 eter. This biotite is biaxial negative with 2 V very small, r < v strong. 

 It is pleochroic in pale greenish brown perpendicular to the cleavage 

 and dark greenish brown parallel to the cleavage. 



The mode of occurrence of this seems to indicate that it is definitely 

 younger than the diabase of normal grain and that it owes its pecul- 

 iar features to concentration of volatile constituents, notably water, 



3J. J. Sederholm. On Synantectic Minerals. Bull. Comm. Oeol. Finland, No. 48, pp. 2-5, 1916. 



