THE MBTAMORPHIC ROCKS OF ADELIB LAND. 8TILLWELL. 25 



it has a massive structure, but if schistose, it is called " diorite gneiss " or " amphibole 

 gabbro schist." If we accepted this nomenclature we should have to give the names 

 " paradiorite " and " diorite gneiss " to two rocks of similar origin but with slight 

 variation in structure. If quartz should be introduced into this rock by some meta- 

 morphic process the rock would become a " para-granodiorite " or a " granodiorite 

 gneiss " ; but we will show that such a rock has nothing whatever to do with 

 granodiorite or its gneissic modification. It is obvious, therefore, that this nomenclature 

 can have little value in any genetic study. 



In the nomenclature that we have adopted we use the term " amphibolite schist " 

 when the amphibolite has a marked schistose structure, the term " biotite amphibolite " 

 when the hornblende is partly replaced by biotite, and the term " quartz amphibolite " 

 when the amphibolite has been modified by the addition of quartz. 



In our study rocks have been found to contain structures similar in appearance 

 to the micrographic and micropegmatitic intergrowths that are formed by the 

 crystallisation of a eutectic mixture in igneous rocks. In numerous instances this 

 " micrographic " intergrowth has a direct metamorphic origin. Consequently, we 

 have not used the term " micrographic " or " micropegmatitic," which may be 

 conveniently retained for igneous structures. We have described the structure a 

 diablastic structure. 



2. FIELD CHARACTERS. 



The Cape Denison granodiorite gneiss is laminated by a rock type which appears 

 as a series of parallel black bands of amphibolites and epidote biotite schists. It is a 

 foliated rock type and its foliation is parallel to the trend of the bands as well as to the 

 foliation of the gneiss. The cleavage planes are usually vertical. The bands are usually 

 between 18in. and 2ft. wide, and being jet black in colour they present strong contrast 

 to the grey gneiss on the bare rock floor. When the gneiss assumes a dark colour relations 

 are less obvious. The junctions between the bands and the grey gneiss are typically 

 sharp, well defined, and straight, and in general appearance they suggest the field 

 relations of a system of parallel dykes which have intruded the gneiss. 



One band may be continuous across the area, but more often it is broken. In the 

 latter case the band wedges out and disappears for a while and then reappears some 

 distance further along the strike. Sometimes a band opens out into a bulge and then 

 occupies a width of 30ft. or more. A case was noted where a band which is continuous 

 for some distance suddenly breaks off and does not reappear along the line of strike, 

 while a band commences in a similarly sudden manner 40yds. due west. A white quartz 

 vein starts from one broken end and runs toward the other, but it peters out before 

 reaching it. One immediately suggests that this band has been faulted, and the fault 

 fracture filled by quartz. But if all the bands were contemporaneously formed, such 

 faulting is highly localised, as the next adjacent band on the east is continuous. 



