456 MR. H. H. READ ON THE [vol. lxxix, 



Quartz, though widespread, is not abundant ; usually it forms 

 interstitial areas between felspar prisms, sometimes it is in rounded 

 grains between which biotite may occur, or takes part in biotite- 

 hornblende-quartz intergrowths. No sign of micropegmatitic 

 intergrowths with felspar has been noted. 



The hypersthene, hornblende, and biotite of the Arnage norite 

 provide an excellent example of what Dr. N. L. Bowen l has 

 recently called a discontinuous reaction series. This series 

 depends upon reaction between the liquid and crystal phases of a 

 solidifying melt, a reaction moving so that an early formed member 

 of the series tends to change over into a later member. A familiar 

 example, and one that has been experimentally investigated, 2 

 is that of the ' reaction -rims ' of rhombic pyroxene around olivine. 

 It is probable that crystallization in the Arnage norite -magma 

 began with the formation of olivine, which, as the temperature 

 fell, was attacked by the liquid to produce Ivypersthene, this in 

 turn to produce hornblende, and the hornblende, biotite ; con- 

 currently, of course, the bulk of the melt had been lessening, and 

 its composition altering by the crystallization of plagioclase, until, 

 when the last of the biotite was being formed by reaction, the 

 silica-residuum crystallized as quartz, often in micropegmatitic 

 intergrowths with the mica. As is seen immediately, the norite of 

 the Glencroft kernel contains olivine, always armoured by hyper- 

 sthene, and so in these norites there is a good example of the 

 reaction series : olivine — hypersthene — hornblende — biotite. 



The rock of the supposed Glencroft kernel differs from the rest 

 of the kernel-norites by containing olivine ; in its other constituent 

 minerals it is exactly like the normal norite just described. The 

 olivine occurs in abundant rounded resorbed grains, always 

 surrounded by a coating (sometimes exceedingly thin) of hyper- 

 sthene. The reaction series : olivine — hypersthene — hornblende — 

 biotite is well seen in this rock. 



A similar rock, an olivine-norite, occurs as a small patch in the 

 contaminated rocks, in the coppice 300 }^ards south of Mill of 

 Kinharrachie. The reaction series described above is especially 

 well developed in this example (fig. 3). 



Chemical composition. — By the kind permission of the 

 Director of the Geological Survey, an analysis of the norite of the 

 Arnage Mass has been made in the Surve}^ Laboratory by Mr. E. 

 G. Badley, to whom my thanks are due. In Table II (p. 457) 

 this and other analyses of gabbro are set forth. The most striking 

 feature of the Arnage norite is its richness in magnesia compared 

 with the other Aberdeenshire gabbros, or with Prof. R. A. Daly's 

 average norite, or with the average of 44 norites enumerated in 

 Dr. H. S. Washington's tables. 



1 ' The Reaction Principle in Petrog-enesis ' Journal of Geology, vol. xxx 

 (1922) p. 183. 



2 Olaf Andersen, ' The System Anorthite-Forsterite-Silica ' Amer. Journ. 

 Sci. ser. 4, vol. xxxix (1915) p. 407. 



