288 ME. W. E. WATT OX THE GEOLOGY [June 1914, 



been described by Mr. A. N. Winchell, 1 which include a ' museova- 

 dite ' or cordierite-biotite-norite. The muscovadite occurs along- 

 the northern line of junction of the gabbro mass with the older 

 schists, and is never found away from the junction and contact- 

 zone. The transition from ordinary gabbro to muscovadite, as well 

 as from muscovadite to mica-schist, proceeds by imperceptible 

 degrees. In the rock described, belonging to the gabbro rather 

 than to the schist series, the four common minerals are cordierite, 

 bronzite, biotite, and quartz. Felspar and enstatite are less common, 

 and the accessoiy minerals are of little importance ; staurolite, 

 however, is present. 



The felspar is more acid than that of the gabbro. being usually 

 a labradorite (Af^AnJ and more rarely andesine (Ab,,An ). It 

 occurs as small grains, and is not very abundant. Bronzite is more 

 abundant than enstatite, and the reddish-brown biotite is an 

 ' anornite.' The cordierite shows polysynthetic bands and also 

 ' trillings.' 



In this case also, Mr. Winchell came to the conclusion that the 

 cordierite-norite was the result of absorption of alumina from 

 external sources — that is, from the surrounding schists. 



In tabular form, the mineralogical characteristics of these rocks 

 and the cordierite-norites of Huntly of the second group are : — 



Le Pallet. Minnesota. Huntly. 



Acid andesine. Labradorite chiefly. Labradorite and andesine. 



Biotite. Anornite. Biotite ? 



Hypersthene. Bronzite and enstatite. Hypersthene. 



Halos in cordierite. Halos wanting. Halos present. 



Almandine. Almandine wanting. Garnet usually present. 



It is greatly to be regretted that no clear sections exist, to prove 

 in the Huntly occurrence a gradual transition from undoubted 

 igneous rock to undoubted phyllite. On the contrary, the junction 

 takes place in cultivated fields, where for a short distance the 

 plagioclase-garnet-biotite-type alone seems to be in situ. To me, 

 however, it appears certain that the rocks of the second type 

 originated from a magma contaminated by the aluminous material 

 of the phyllites, as at Le Pallet and in Minnesota. 



In the case of the cordierite-norites of the first type, some 

 absorption of material by the xenoliths and the intruded rocks 

 has certainly taken place. The slices of cordierite-norite from 

 Battlehill show both quartz and orthoclase ; orthoclase occurs in 

 the cordierite-norite of Ordiquhill. Such transference of material, 

 however, seems to be slight, and I am inclined to ascribe to the 

 heat of the later intrusion the chief part in bringing about 

 the formation of these cordierite-norites. With differences in 

 temperature and differences in the amount of material trans- 

 ferred, the various types have been formed. The chief chemical 

 changes produced have been an increase in potash and a decrease 

 in the amount of lime. 



1 ' Mineralogical & Petrographical Study of the Gabbroid Rocks of Minne- 

 sota ' Am. Geol. vol. xxvi (1900) pp. 294 et seqq. 



