BT W. N. BENSON. 351 



metamorphosing solutions, which have replaced the basic felspar 

 by albite and minutely granular quartz-mozaic. 



This correlation would involve some alteration in the details 

 of the tectonics and stratigraphy assumed previously for the 

 neighbourhood of East and West Gap Hills (see below). 



7%e Albite- Dolerite. — This zone commences to the west of the 

 keratophyre in Portion 37, and occurs first in a series of small 

 isolated lenses running south-east until the first great cross-fault 

 is reached. No more appears for half a mile to the south of 

 this, but, beyond the second cross-fault, a large sill appears, and 

 continues thence without a break across Cope's Creek to Pipeclay 

 Creek, a distance of over two miles. The sill is nearly 200 yards 

 in width, and its dip is approximately vertical. At its southern 

 end, the dolerite is faulted against the eastern mass of the Silver 

 Gully agglomerate, and a great ridge of coarsely crystalline, 

 ferruginous jasper has developed in the plane of movement. (The 

 appearance of this ridge would lead one to expect that a little 

 ferruginous keratophyre would be found along its western face. 

 Time has not permitted the writer to search for this). 



Petrologically, this zone is rather varied; a specimen taken 

 from the northern portion shows a mixture of sub-ophitic and 

 sub-variolitic textures, with clear albite, and augite (1378); but 

 west of Black Jack, in the main zone of rock, a dolerite occurs 

 (1376), the albite of which is very dusty and associated with 

 prehnite, and has evidently been derived from a more basic 

 felspar, with associated alteration of the augites to chlorite. 



This zone of dolerite, lying as it does between the Serpentine 

 Line and the Nemingha Limestone, is probably to be correlated 

 with the dolerites that extend from the mouth of Sheep Station 

 Creek (three miles south-east of Pipeclay Creek) to beyond 

 Hanging Rock {see 3, Plate xxi.). We may also consider that it 

 finds a repetition in the zone of dolerites and spilites that lie 

 west of the Nemingha Limestone, and extend from near Bowlino- 

 Alley Point across Moonlight Hill and Tom Tiger into Swamp 

 Creek. In these, many examples of the sub-ophitic to sub- 

 variolitic albite-dolerites have been observed (5, PI. xxv., fig. 2). 



