part 3] MiNOE iNTErsiONs ik the island op mull. 249 



18 per cent., the remaining constituents entering into the compo- 

 sition of the glass, which holds a fairly large amount of water in 

 solution. Such rocks are well known from other localities, and are 

 produced artificially every day in glass-pots and furnace-nozzles by 

 the fusion of the aluminous material of which they are constructed. ^ 



The miiformity observed in the aluminous xenolithic material 

 makes it clear that we have only one type of aluminous sediment to 

 consider, and that this was in all probability of wide distribution. 



One somewhat unusual feature brought out by Mr. Radley's 

 analysis is the excess of soda over potash. In most clays the 

 reverse is the case, but exceptions are not unknown. 



(iv) Relation of the Xenoliths to the Enclosing Rock. 



A striking feature connected with the xenoliths is the extreme 

 sharpness with which the}^ are marked off from the enclosing rock, 

 and the unmodified character of the igneous rock in contact with 

 them. It is clear without additional evidence that the meta- 

 morphism was not effected in place by the igneous magma now 

 represented by the surrounding tholeiite. but was carried out at 

 much greater depths and at a more elevated temperature than that 

 appertaining to the rock at the time of its intrusion. 



The most convincing evidence of deep-seated metamorphism, 

 however, is furnished by the fact that the enclosing rock often bears 

 no relation to the various metamorphic zones of the xenoliths, and 

 therefore cannot have produced them. It has been pointed out 

 (p. 241) that these zones are occasionally concentric and roughly 

 spherical in form, more often they occur as crescentic or straight 

 parallel layers ending abruptly at the surface of a xenolith. It is 

 abundantly clear, as we shall see, that these zones are the result of 

 progressive metamorphism, and yet in the portion of the sill that 

 we can study we find the enclosing rock crossing from one meta- 

 morphic zone to another — from sillimanite-buchite to the anorthite- 

 sapphire-spinel-layer — without any corresponding change in its 

 texture or composition, or any diminution in the sharpness of the 

 line of junction. No better evidence is needed to prove that the 

 metamorphism of the xenoliths was practically complete before they 

 were carried by the molten igneous rock into their present position. 



That the xenoliths were still plastic and partly molten at the 

 time of their intrusion is shown by their often intensely vesicular 

 character. These vesicles occur in the now glassy portion of the 

 xenoliths, and mark the escape of gas from the molten material on 

 the intrusion of the xenolith from a region of high to a region of 

 lower pressure. 



Prof. A. Lacroix has pointed out, from his study of xenoliths, 

 that the mutual chemical changes produced by a magma on 

 accidental inclusions and vice versa are usually quite limited ; and, 

 therefore, he rightly asserts that, in order to produce any con- 

 siderable effect, a prolonged sojourn of the xenoliths within the 

 molten magma is necessitated. 



^ G. V. Wilson, op. jam cit. p. 200 ; A. H. Cox, ' Notes on some South 

 Staffordshire Fire-clays & their Behaviour on Ignition ' Geol. Mag. 1918, p. 59. 



