474 ME. H. H. BEAD ON THE [vol. lxxix, 



country -rock (see figs. 2 & 5, pp. 452, 468) ; examples have 

 already been described from Towie Wood (p. 459), Ardlethen 

 (p. 469), and elsewhere. The process of lit-par-lit injection 

 does not markedly disturb the lie of the xenoliths. In the case of 

 the smaller xenoliths, no definite orientation is to be seen, except 

 that their longer axes agree in direction with the fluxion when 

 this is at all well expressed. 



In size, the xenoliths vary from yards to fractions of an inch 

 across, but an average size for slaty xenoliths is about 3 inches 

 long by 1 inch wide, and about half an inch thick. 



The slaty xenoliths, and those of the hornblende-schist produced 

 by the coalescence of the Kinharrachie veins (see p. 467), are 

 rounded in form ; but quite different are those of quartzite of the 

 Fy vie Series, which are angular and have sharp, straight edges. It 

 is instructive, too, to compare the sizes of the argillaceous and 

 quartzose xenoliths occurring in the same rock-exposures of the 

 Arnage Type : — 



Measurements in Inches. 



(1) Gallowhill.— Quartzose xenoliths: 12x3; 6x2; 7x3; 16X6; 

 24x4. 



Argillaceous xenoliths : 2x1; 4x2; 2 X i ; 2 X J. 



(2) South of Waulkmill of Savoch. — Quartzose xenoliths: 7x4; 

 3X3; 9X1; 2±Xf ; 10x6. 



Argillaceous xenoliths : faint ' ghosts ' only seen. 



(3) Towie Wood.— Quartzose xenoliths: 23x6^; 24x6; 7x4; 3xU; 

 6X3. 



Argillaceous xenoliths : 11x3; I2XI3; 4x1; and many smaller. 



The conclusion appears to be warranted that argillaceous 

 sediments are more readily reacted upon by the initial magma 

 than is quartzite. The same result is seen in the Ardlethen roof- 

 cake (see p. 469), which is composed of quartzite, but floats upon 

 and is veined by contaminated rock carrying argillaceous xenoliths. 

 We see too, perhaps, an expression of the same phenomenon in the 

 nesting of the contaminated rock bj quartz, which seems to have 

 been rejected and segregated in the contamination -process. It 

 may be concluded, then, that the most important contaminator 

 for the Arnage Type of product is sediment of argillaceous nature. 



The only xenolith observed in the norite-sheet may here be 

 noted ; it consists of quartzite, and stands up wall-like in the 

 norite in the railway-cutting south of Burngrains. The norite is 

 norite and the quartzite quartzite up to the very junction-planes. 



Petrography. 



The hornfelses of the Arnage district are of such beauty that it 

 is considered advisable to make them the subject of another com- 

 munication. Enough will be said here, however, to enable the 

 reader to form an idea of their nature and composition, since they 

 play an important part in the contamination-process. The 



