granular, probably or certainly pegmafoid phases of the dolerite, a rather exceptional case being that described by Emerson in which the pyroxenes have assumed a curiously “plumose” type of spherulitic structure, and occur in a coarse-grained, but irregular narrow band near the margin of a main intrusion. In one case only there is a suggestion of a shearing movement in the consolidating rock; usually there is no evidence of this. Tomkieff's (op. cit. pp. 116–8) conclusion concerning similar but more sharply bounded coarsegrained dolerite in the Whin Sill, namely, that it was “possibly the result of ‘wet’ differentiation formed in the intercrustal basin, caught up in the ascending magma and stretched out in the form of lozenge-shaped tabular bodies parallel to the walls of the injection chamber” is not applicable to this rock-mass, which seems to have formed by differentiation in situ. (See Shannon, 1924, p. 39, as quoted below p. 111, and Phemister, 1928, pp. 162–170). In general, the vesiculation and abundance of deuteric minerals throughout this sheet indicates the abundance of fluxing volatile materials in the invading magma, and the consequent facilitation of its gravitative differentiation to a greater extent than occurs in the much thicker intrusive sheets of the Palisades (Walker, 1941), the Karroo (Walker, 1940, etc.), Tasmania (Edwards, 1942) or (so far as is known) Antarctica (e.g., Benson, 1916).* We may contrast these products of magmas not rich in fugitive constituents with the more deuterically altered and gravitationally differentiated diabase sill at Bridgehead, Ontario, which presents interesting analogies and contrasts with the N.E. Otago Sheets (Emmons, 1927, pp. 73–82); see also Phemister (1928, pp. 102–170, 185). The need of fluxing to promote gravitative differentiation in sills was stressed by Harker (1916, p. 555), who noted that “clear instances of gravitative differentiation in sills and laccolites … are all in rocks which must represent very unusually fluid magmas such as the analcime-bearing intrusions of Permian Age in Scotland,” and, we may add, the richly zeolitic theralitic intrusion at Waihola (Benson, 1942). The bulk of the water in the Tawhiroko, Main Moeraki and Mt. Charles intrusive sheets was probably magmatic, but this might have been supplemented by water derived from the plastic sediments which they invaded (cf. Leitmeier, 1909; Daly, 1917, p. 445, 1933, pp. 307–11; Shannon, 1924, p. 39), a view which Grout (1928, pp. 567–70)§ But see Grout (1932, pp. 212–3). opposed, though Day and Allen's (1925, pp. 76–84) application of Morey's (1922) experimental study of the absorption of water into silicate melts at low (even atmospheric) pressures of “meteoric water acquired in the usual way through contact with water-bearing strata or reaching the volcanic hearth under a head determined by the elevation of the crater basin,” to the explanation of the effect of downward percolating snow-water in stimulating the eruptive activity at Lassen Peak, supports this view. The general course of the formation of these Otago Sills has, however, much in common with that of the large Palisade Sill (Walker, 1940, p. 1101). Thus:— 1. “The main differentiation was effected by the sinking of early-formed olivine followed at a later stage by pyroxene.” The