262 



AUSTRALASIAN ANTARCTIC EXPEDITION. 



This usage does not conform with the definition issued by the Joint Committee of 

 the Geological Society of London and of the Mineralogical Society in the recent report 

 on British Petrographic Nomenclature 1 . This Committee retain the term amphibolite 

 for unfoliated or slightly -folia ted metamorphic rocks of doubtful or other than igneous 

 origin. It is composed essentially of hornblende and felspar, often containing various 

 accessories, such as epidote and garnet. They distinguish hornblende-schist from 

 amphibolite by the possession of a foliated texture. 



On this basis completely recrystallised rocks, which consist of hornblende and 

 felspar, and which possess a slightly-foliated structure, are excluded from the family 

 of amphibolites when they are derived from igneous rocks. It would appear to be the 

 intention of the Committee to include such cases among the epidiorites which are defined 

 as unfoliated basic igneous rocks in which the augite is completely altered to 

 hornblende. They seem to have overlooked the fact that two types of altered basic 

 hornblende rocks, occurring in the form of a dyke or sill, can be recognised, (1) those 

 in which the ferro-magnesian has suffered recrystallisation, and (2) those in which 

 both the ferro-magnesian and felspar have suffered recrystallisation. These two 

 types may be encountered in one area, as at Broken Hill (New South Wales), where 

 they are readily distinguishable from one another in the field, in the hand specimen 

 and under the microscope. The term epidiorite covers the first type, and it is clearly 

 undesirable to extend it to the second type, which is identical with amphibolite. 



A typical amphibolite will contain 60 to 70 per cent, of hornblende and about 

 25 to 30 per cent, of felspar. Other metamorphic rocks occur which contain nearly 

 all hornblende and practically no felspar, and it is necessary for precise description 

 to distinguish these from the typical amphibolite. It is also desirable to distinguish 

 between the foliated amphibolite and the hornblende schist with felspar in excess of 

 the hornblende. These metamorphic types, in which hornblende is an important 

 constituent, may be tabulated as follows : 



Mineral composition. 



With massive structure. 



With foliated structure. 



Mainly hornblende 



Hornblende in excess of felspar... 

 Felspar in excess of hornblende 



Hornblende fels 



Amphibolite 



Hornblende-plagioclase-gneiss 



Amphibole or hornblende-schist. 

 Amphibolite- schist or mica -amphibolite 

 Hornblende-plagioclase-schist. 



The massive types pass by gradual transitions into the foliated types. It is 

 obviously difficult to restrict the term hornblende-schist to foliated amphibolites, as 

 recommended by the Joint Committee. It is more logical to use the term for the pure 

 amphibole types as has been done by Grubenmann 2 . The term amphibole-schist 

 includes rocks which contain more than one variety of amphibole, but those which 

 contain anthophyllite are referred to as anthophyllite schists in the following pages. 



" Report on British Petrographic Nomenclature." Min. Mag., Vol. xU, No. 82, pp. 137-147, 1921. 

 " Die Kristallinen Schiefer." U. Grubenmann. Berlin, 1910, p. 216. 



