Reference to some earlier literature on the clouding of plagioclase is given in the recent paper by Polderwaart and Gilkey (1954). Polderwaart and Gilkey discuss the origin of the clouding material in different rocks, and arrive at the conclusion that the heavy dust shown by some plagioclases cannot reasonably be explained by exsolution. The clouding material, by most authors supposed to be iron ore, must be sought for outside the plagioclase.
This study is confined to the plagioclases in the coronited metadolerites of two dolerite provinces of Norway, i.e. the dolerites (hyperites) in the Precambrian of Southern Norway described in the classic paper by Brögger (1936), and the dolerites in the Caledonides of Western Norway which have been the object of a detailed investigation by T. Gjelsvik (1952).
An examination of the literature shows that clouding is found to be more frequent in dolerites and allied rocks than in all other rock groups together, and that the plagioclase of coronites invariably contains dustlike inclusions.
In the hyperites of Sweden the plagioclase is so commonly pigmented with a fine brown dust that Hjelmquist (1949) includes this characteristic in the definition of the hyperites.
In his paper T. Gjelsvik repeatedly calls attention to the clouded aspect of the plagioclasc (p. 49, 55, 71, 98, 99, 103).
The present writer encountered swarms of meta-clolerite lenses during field work in the Vestranden gneiss area in 1953 and, together with I. Hernes, in Nordmøre last summer. (Hernes 1955). Gjelsvik and professor dr. J. A. W. Bugge have kindly placed their slide collections at my disposal.