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



(c) Cordierite-Norites and their Allies. 



In the Huntly area the rocks of this t}q:>e can be placed in one 

 of two groups, according to their mode of origin. They are found 

 under different field-conditions, and the same mode of origin cannot 

 be advanced to explain the two occurrences. Here the larger group 

 is composed of rocks, originally norites, that have been altered by 

 the subsequent intrusion of the rocks of Carvichen and Ordiquhill. 

 The resulting cordierite-nOrites occur, therefore, where the older 

 norite is pierced by these later intrusions ; but, at the same time, 

 it is evident that contact-metamorphism alone is not responsible 

 for the whole change — some transfer of material has certainly taken 

 place. The second and smaller group occurs along the western 

 margin of the newer intrusion, at the junction with the crystalline 

 schist. Unfortunately, the boundary is in a cultivated area, and 

 it is an allied type only that seems to occur in situ. 



In both groups, however, the mineral composition is similar : 

 cordierite, hypersthene, biotite, garnet, and felspar being present. 

 Hypersthene, usually present and necessarily so in the norites 

 proper, is, as a rule, strongly pleochroic, is schillerized, and may 

 be heavily charged with impurities. The hypersthenes of the 

 first group are, on the whole, lighter in colour than those of the 

 second. In the latter there is a tendency to form roughly rect- 

 angular grains with a slight linear arrangement. There is a 

 widespread association of the hypersthene with garnet, magnetite, 

 biotite, and pleonaste. 



Great variation is noted in the felspars of these rocks. The 

 basic bytownite has now vanished, and the plagioclase is represented 

 by labradorite (/i = l'559) or andesine (/x = l*551) ; while from the 

 later intrusions orthoclase itself may have passed over. In the 

 formation of this type, the ophitic structure has been broken down, 

 and the felspars are often found as groups of small grains, associated 

 generally with pleonaste. 



Biotite, occurring in practically every slice, may be developed to 

 the complete exclusion of hypersthene. For its formation the 

 cordierite-norite is indebted for potash to the intrusive rock. When 

 pyroxene is present, the two minerals are closely associated, and the 

 biotite is often corroded, forming intergroAvths with such minerals 

 as quartz, orthoclase, and cordierite. 



The cordierite of these rocks contains many zircons surrounded 

 by a characteristic halo. Twinning is common, giving rise to 

 lamellar twins and those of the penetration type (drillings- 

 bildung), showing 'sector-polarization.' In the rocks of the 

 Huntly area, twinning is far more abundant in rocks of igneous 

 origin — the cordierite-hornfelses rarely show twinned cordierites, 

 and never those giving ' sector-polarization.' The restricted occur- 

 rence of such twinning was noted by Dr. J. J. H. Teall x ; but 



1 ' Natural History of Cordierite & its Associates ' Proc. Geo]. Assoc. 

 vol. xvi (1899-1900) p. 62. 



