256 DE. H. H. THOMAS O^^ XEXOLITHIC [vol. Ixxviii, 



ranged from near 1400° to 1250° C, the lower temperature 

 approximating to that at which the magma, together with its 

 xenoliths, was intruded into its present position. 



There is practicalh^ no evidence such as would lead to the 

 conclusion that the magma as a whole has been modified to 

 any extent by assimilation of sedimentary material, despite the 

 fact that certain intrusions consist almost entirely of xenolithic 

 matter. 



EXPLANATION OF PLATES VI-X. 



[The numerals preceded by S are tlie registration-numbers of tbe respective 

 rock- slices in the collections of the Geological Survey.] 



I am indebted to Mr. John Rhodes for the preparation of the photomicro- 

 graphs which are reproduced in these plates. 



Plate VI. 



Fig. 1. Glassy acid interior of a composite sill, Eudh' a'Chromain. Ehyolite 

 allied to inninmorite. Blade-like and acicular crystals of augite 

 and small porphyritic crystals of acid labradorite in a pale 

 lavender-brown glassy matrix. Local devitrification has given 

 rise to patches of acid labradorite, showing white in the figure. 

 S. 18486. X 16-5. (See p. 235.) 



2. Acid interior of a composite sill, Eudh' a'Chromain, more or less 



completely devitrified. The more complete devitrification has 

 caused the separation of less basic plagioclase, together with some 

 orthoclase and quartz. S. 18488. X 18. (See p. 235.) 



3. Tholeiite. Upper edge of the basic portion of a composite sill, 



Eudh' a'Chromain. Narrow lath-shaped crystals of labradorite and 

 chloritized augite, in a brown glassy matrix charged with magnetite. 

 S. 18480. X 18. (See p. 235.) 



4. Tholeiite. Basic stony portion of a composite sill, Eudh' a'Chro- 



main, showing the characteristic structure assumed by these rocks. 

 The constituents are labradorite and hypidiomorphic augite, with 

 a relatively small amount of residual glass. S. 18490. X 18. 

 (See p. 235.) 



5. Cognate xenolith, in the lower basic portion of a composite sill, 



Eudh' a'Chromain. The xenolith consists of glomeroporphyritic 

 bytownite and greenish augite. The enveloping tholeiite is similar 

 to that shown in fig. 4. S. 17173. X 15-5. (See p. 238.) 



6. Cognate xenolith, in the lower basic portion of a composite sill, 



Eudh' a'Chromain. The xenolith consists of bytOAvnite and hyper- 

 sthene. The hyper sthene is sharply idiomorphic and intensely 

 pleochroic. The felspars are idiomorphic, of large size, moulded 

 on the hypersthene, and their composition is that of basic labra- 

 dorite or bytownite. S. 16598. X 18. (See p. 238.) 



Plate VII. 

 Accidental Xenoliths. 



Fig. 1. Sillimanite-buchite, 3500 feetnorthof westof Ormsaig. Needles 

 of pale-pink sillimanite, embedded in clear pale lavender-coloured 

 glass. Distinct fluxion- structure is developed locally. The rock 

 is slightly vesicular, the vesicles being filled with zeolites and 

 calcite, and results from the simple fusion of an aluminous shale. 

 S. 18005. X 36. (See p. 243.) 



