BY W. N. BENSON. l25 



tions; and, moreover, pillow-lavas have been found associated with 

 lacustrine deposits(36). Jukes-Brown has argued for the shallow- 

 water origin of the radiolarian rocks associated with the pillow- 

 lavas of Ballantrae in Ayrshire(ll), and Professor David and Mr. 

 Pittman have declared that the Tam worth radiolarian claystones 

 etc., which are continuous with those associated with the pillow- 

 lavas of the Nundle district, were developed in comparatively 

 shallow water(12). 



The mode of occurrence of special masses may now be described. 

 No doubt can exist as to the intrusive character of any of the 

 large patches of dolerite marked on the geological map of the 

 district given in Part ii. They all represent areas of dolerite of 

 coarse or medium grainsize, and almost free from vesicles. Not 

 infrequently there occur in them veins of very coarse-grained 

 dolerite-pegmatite. Their intrusive contacts with the surround- 

 ing sediments are clearly observable. Often they have them- 

 selves been invaded by later masses of dolerite, which have a 

 strongly marked, fine-grained, chilled, marginal zone against the 

 invaded rock. Such contact-zones may be seen in several places 

 in the lower part of Munro's Creek. 



The areas marked as spilite-flows require more detailed con- 

 sideration. The most important is that crossing the Peel River 

 two miles south of Bowling Alley Point, and extending thence 

 towards Hanging Rock. In the cutting on the main road, it 

 consists of a group of stratiform masses, which may be seen to 

 have intrusive contacts with the radiolarian claystones on either 

 side. The same features are to be seen where this zone crosses 

 Madden's, Moonlight, and Daylight Creeks, south of this point. 

 The individual sills have a medium to fine grainsize, and are 

 often amygdaloidal. In microscopical texture, they vary between 

 ophitic dolerites and variolitic spilites : this variation may be 

 seen on either side of a sill, and is due to marginal cooling; 

 frequently, however, the sill is of uniform texture throughout. 

 Multiple sills occur here and there, with chilled edges between 

 the component parts. The vesicles are filled with calcite, epidote, 

 and chlorite, and along the northern slope of Tom Tiger, axinite 

 is frequently present in the vesicles, while axinite- and quartz- 



