part 4] THE PETROLOGH' OF THE ARNAGE DISTRICT. 453 



andesine to labradorite. A few grains of black iron-oxide complete 

 the rock. 



In summary, then, the contaminators of the Arnage Mass 

 are of four petrographic types : — 



1 . An dalusite-cordierite- schists . 



2. Felspathic quartzites. 



3. Biotite-oligoclase-g-neisses and biotite-schists. 



4. Hornblende-schists. 



The contact-metamorphism of each of these rock-types is 

 described in § VI, pp. 473-78. 



IV. The Norite Sheet. 



Before considering the rocks to which the name contaminated 

 is here applied, we may deal with a very striking feature of the 

 Arnage district, and that is the norite sheet which underlies 

 the zone of contaminated rocks and into which they pass down- 

 wards (see fig. 2, section, p. 452). H. Rosen busch l said, 

 concerning the norite, that it was an extraordinarily beautiful 

 representative of its class. Dr. A. Harker ~ has figured a typical 

 norite from Towie Wood. In the 1886 Geological Survey Memoir 

 the norite (and the contaminated rocks) were all designated 

 ' diorite ', though, it is true, the variations of the ' diorite ' were 

 there noted as considerable. 3 



Distribution. — The distribution of the norite is a striking one 

 (see fig. 2). The country is of low relief, but even so it is fairly 

 obvious that the norite is seen at slightly lower levels than the 

 contaminated rocks. It forms small areas exposed by the removal 

 of the overling contaminated-rock zone, and surrounded by these 

 contaminated rocks. If erosion had proceeded to a slightly less 

 extent, it is probable that none of the norite would have been 

 exposed. These norite outcrops do not represent cupolas — they 

 are best regarded as inliers, as it w r ere, of a somewhat level-topped 

 norite sheet (see fig. 2, section, p. 452) ; they ma} r be called, for 

 the sake of simplicity, kernels. 



Five kernels have been recognized ; they are, from north to 

 south, those of Inkhorn, Waulkmill of Savoch, Arnage, Little 

 Arnage, and Glencroft. 



The Inkhorn kernel has a longer axis three-quarters of a mile 

 long ; its breadth is probably less than half a mile. The rock of 

 this kernel is well exposed in the railway-cutting north and south 

 of Mill of Inkhorn, and the limits of the norite can be fixed at 

 both ends, for there is a somewhat abrupt passage into contami- 

 nated rocks. 



1 ' Mikroskopische Physiograi^hie ' 4th ed. vol. ii, pt. 1 (1907) p. 349. 

 - ' Petrology for Students ' 5th ed. (1919) pp. 77-78 & fig. 23 B. 

 3 J. S. Grant Wilson, ' Explanation of Sheet 87 ' Mem. G-eol. Surv. Scot. 

 1880, p. 17. 



