454 Mil. If. JI. HEAD ON THE [vol. lxxix, 



The Waulkmill of Savoch kernel is small, being less than a -third 

 of a mile long, and is situated astride the Ebrie Burn. Good 

 examples of the rock of this kernel are seen in the railway-cutting 

 at Waulkmill of Savoch, near which locality, too, the associated 

 contaminated rocks may be studied. 



The third kernel, that of Arnage, occurs south of Arnage 

 Station ; normal norite can be examined in the railway-cutting, 

 and its contaminated associates observed with some clearness on 

 the brae east of the Ebrie Burn, east of Mains of Drumwhindle ; 

 this kernel is about a third of a mile in greatest diameter. 



The Little Arnage kernel is found farther south again, and is 

 exposed also in the railway-cutting ; its length is about two-thirds 

 of a mile, and it exhibits good contamination phenomena on the 

 south. 



The last kernel, that of Glencroft, on the northern bank of the 

 Ythan, is somewhat unsatisfactory ; here is an abundance of 

 large blocks of norite, some of which are probably in place. The 

 argument, however, is not affected, even if the Glencroft kernel 

 be neglected. 



Taken together, the norite kernels cover an area of Avell under 

 one square mile. 



Petrography. — The norite is a homogeneous fairly coarse 

 rock, which weathers either into rude spheroidal masses, or into 

 rounded knob-forms coated with a thin skin of decomposed 

 material. In the kernels the rock appears thoroughly uniform. 

 With a lens, felspar, a dark mineral (hypersthene), and scarce 

 biotite-plates can be recognized. The rock is quite fresh. 



About twenty slices of the norite have been cut. All the 

 kernels, with the exception of the doubtful Glencroft one, show 

 exactly similar norite. The rock is composed almost entirely of 

 labradorite and hypersthene (see tig. 3, p. 455) ; quite subordinate 

 are hornblende, biotite, diallage, quartz, iron-oxide, and apatite. 



The felspar is fresh, and forms subhedral or euhedral prisms, 

 often l - 25 mm. long : its sign is positive ; albite and rare pericline 

 twinning is seen ; its optical properties indicate that it is a 

 labradorite of the composition Ab, l5 An^. Sometimes the interiors 

 of the crystals are slightly more basic than the exteriors. The 

 felspar often includes multitudes of minute brownish-red plates 

 which have apparently no definite orientation with regard to 

 their host, although among themselves they are arranged on three 

 planes. 



Hypersthene forms prismatic crystals, often 1 mm. long by 

 05 mm. wide. Cross-sections show the dominance of pinacoids 

 over prism faces, and the usual prismatic cleavage. The pleochroism 

 is striking — a pale pink, ft pale yellow-green, c pale green ; the sign 

 is negative. The mineral usually is veiy thoroughly schillerized. 

 An intergrowth on a minute scale with a monoclinic pyroxene is 

 occasionally seen. Often the hvpersthene-crystals exhibit the 

 svnneusis structure of Voy't. bemt> - collected into swarms and 



