part 3] Mi]S"OR intrusions in the island of mull. 247 



of the cordierite so formed to spinel is determined by the relative 

 amounts of magnesia and silica available. If there be an excess of 

 silica, cordierite would form alone ; but, with a deficiency in silica, 

 spinel would be its necessary companion.^ 



Spinel. — The spinellids of the xenoliths, taken collectively, 

 present a considerable variety of colour and, presumably, of compo- 

 sition ; but, considered in connexion with their respectively-associated 

 minerals, they show a fair amount of consistency. The chief mode of 

 occurrence of spinel is as a deep-green variety (hercynite-pleonaste), 

 in intimate association with anorthite in the external crystalline 

 zone of the xenoliths. The manner of association of these two 

 minerals indicates contemporaneous crystal-growth, with a gradual 

 convergence towards eutectic composition. Such a condition api^ears 

 to have been actually realized in some instances under special con- 

 ditions, and a highly characteristic structure has resulted (PL YIII, 

 iig.o). In this association the spinel is usually without well- 

 defined crystalline boundaries, and its form is almost exactly similar 

 to that assumed by olivine, when associated with anorthite in such 

 rocks as the allivalites of Dr. A. Harker.^ 



It has presumably been formed through the solution of sillimanite 

 and an excess of alumina of the sillimanite-buchite by a magma 

 locally enriched with lime, ferrous iron, and, to a less extent, 

 magnesia. An analj^sis of this early-formed leek-green sj)inel, 

 separated from the anorthite-corundum-spinel assemblage of a 

 xenolith from Locality 2, is given below, and indicates a variety 

 lying between the magnesian pleonaste and the ferrous hercynite. 



Analysis of Dakk-Green Spinel from Xenolith (Anal. Ill, p. 236), 

 Nuns' Pass, bt E. G. Kadley. 



I. II. 



Per cent. Per cent. 



Si02 0-77 — 



Ti02 OoO — 



AI2O3 60-84 61-70 



FeoOs 4-26 4-32 



FeO 24-UO 24-34 



MnO 0-15 0-15 



CaO 0-36 0-36 



MgO 9-37 9-50 



H2O above 105° C. ... 0-14 — 



Total s 100-3 9 100-37 



The figures given in tlie second column are dei'ived from those of the first by 

 recalculation, after rejecting silica, titanic oxide, and water, which are regarded as 

 non-essential constituents. See ' Suminary of Progress for 1913 ' Mem. Geol. Surv. 

 1914, pp. 80, 81. 



A very beautiful relation between spinel and anorthite may 



'■ G. A. F. Molengraaif, ' Cordierit in einem Eruptivgestein aus Siidafrika ' 

 Neues Jahrb. vol. i (1894) p. 79 ; see also J. Morozewicz, ' Experimentelle 

 Unterstichungen iiber die Bildung cler Minerale im Magma ' Tscherm. Min. 

 Petr. Mitt. vol. xviii (1898-99) pp. 56, 57. 



2 ' The Natural History of Igneous Rocks ' 1909, fig. 49, p. 170. 



S2 



