368 H. H. Read — The Two Magmas of Strati hbogie. 



Series, and as will be seen later at Huntly among the Younger 

 Series, it can be shown that differentiation lias not taken place 

 in situ. The absence of gravitative separation and of continuous 

 variation and the presence of independent monomineralic intrusions 

 point to the conclusion that differentiation took place in a lower 

 magma basin. The main intrusion was undoubtedly one of 

 a heterogeneous, 1 predominantly gabbroic, magma, and from this 

 magma had already been separated off the two monomineralic types 

 — pyroxenite and anorthosite. On Bowen's theory, such separation 

 of monomineralic types would be by the collection of masses of 

 crystals of pyroxene and of labradorite, and therefore, if such 

 monomineralic rocks were intruded as independent masses, their 

 origin would be demonstrated by cataclastic structures. Such 

 structures are, of course, apparent in the monomineralic rocks of 

 Portsoy, but these rocks were affected by the regional folding and, 

 in common with all the igneous rocks of the Older Series, show 

 evidences of crushing stresses. How, then, are these crush effects 

 due to later folding to be distinguished from those due to intrusion 

 as a mass of crystals ? Orientation of crystal plates has never been 

 seen in the massive pyroxenites of this series, there is no segregation 

 of olivine into layers, and the crystals of pyroxene form interlocking 

 junctions. The sporadic olivine is often interstitial to the pyroxene. 

 In the author's opinion, the crushing seen in the pyroxenite is best 

 explained as due to the effects of regional folding on a solid and cold 

 rock. Similarly the crushing of the anorthosite is exactly like that 

 of the augen-gneisses. Until further knowledge is obtainable 

 concerning the immiscibilit)^ of silicate magmas, 2 especially under 

 pressure, and also concerning the role of mineralizers, particularly 

 of magmatic water, in ultrabasic rocks, the question of the origin of 

 the monomineralic rocks of the Older Series must be left open. On 

 the whole, however, the writer considers some kind of separation of 

 a magma into liquid phases to be demanded by the field relations of 

 the Older Series of this district. Such a separation of liquid phases 

 would be afforded by the operation of liquid immiscibility, or by the 

 refusion of a solidified magma basin already differentiated, perhaps 

 by crvstallization. Such a refusion was postulated by Martin 

 Schweig 3 and has lately been favoured by Dr. Harker, 4 but the 

 cause of the rise in the isogeotherms evoked by the latter is 

 unexplained. Liquid immiscibility, therefore, until proved 

 impossible, should be considered as a process probably effective in 

 magmatic differentiation. 



2. The Youngek Seeies. 

 The Younger Series of the Strathbogie district forms an elongated 

 mass with a north and south trend, a direction parallel to the 



1 Cf. Geikie & Teall, " On the Banded Nature of some Tertiary Gabbros in 

 the Isle of Skye " : Q.J.G.S., vol. 1, p. 656, 1894. 



2 Cf. R. A. Daly, Igneous Bocks and their Origin, 1914, pp. 225, 226. 

 " Genesis of Alkaline Rocks " : Joum. Geol., vol. xxvi, pp. 123, 124, 1918. 



3 M. Schweig, " Untersuchungen iiber Differentiation der Magmen " : Neues 

 Jahrbuch, Beil. Bd. xvii, p. 563, 1903. 



4 A. Harker, "Differentiation in Intercrustal Magma Basins": Joum. 

 Geol., vol. xxiv, p. 556, 1916. 



