BY G. D. OSBORNE. 177 



(h) Volcanic conglomerate, etc. This is the horizon described in detail in 

 the Langlands Section. Here there are some features which are unique, particu- 

 larly the occurrence of bands of coarse breccia, many of the fragments simu- 

 lating the appearance of varves. It is just possible that these fragments have 

 been derived from the underlying cherty rocks {g). The secondary quartz is 

 abundant, and much of the interstitial matrix is stained a bright green colour, 

 due to intiltration of iron compounds. Thickness, 90 feet. 



(i) Felsite. This is the third and last massive rock in the section. It is 

 best developed a little to the west of the line of traverse, forming part of the 

 rough timbered hill near the Black Rocks. The rock is porphyritic in tiny red 

 felspars, probably orthoclase, and a little free quartz, the dense groundmass 

 predominating. Thickness, 60 feet. 



(j) Conglomerate, etc. This unit is composed of a series of bands of 

 gritty pebbles and some brecciated material similar to that observed with the ag- 

 glomerates. Thickness, 130 feet. 



The total thickness of the rocks described in the Oakendale Section, omit- 

 ting the conglomerates mentioned in the preliminary paragraph, is 620 feet. 



The Volcanic Stage at Martin's Creek. 



The basal portion of the Volcanic Stage at Martin's Creek, has already 

 been described (p. 168). There is a variety of rocks of the Volcanic Stage out- 

 cropping around the village of Martin's Creek, and between that locality and 

 Paterson. The description of these rocks given by Mr. Sussmilch (Sussmilch and 

 David, 1919, pp. 262-266) can only be regarded as of a general nature. Of the 

 four distinct flows referred to by the field names of dacite and rhyolite, the 

 lowest, considered stratigraphically, is an important biotite-quartz-keratophyre. 

 This is identical with the rocks from about the same levels in the other sections, 

 but is not repeated in the sequence at Martin's Creek, as is the case in most of 

 the other localities. The nature of the second flow is hard to discern under the 

 microscope, but albite is certainly present in a devitrified base, and it is possible 

 that the rock is a soda-rhyolite. The third flow, outcropping on the road be- 

 tween Gostwick Bridge and Mt. Johnstone, is a da«ite, but elsewhere to the 

 south-east becomes a toscanite. The fourth unit has been described by W. R. 

 Browne (Sussmilch and David, 1919, Appendix ii.), the daeitie nature of the mas- 

 sive portions of the flow being demonstrated. Associated with this dacitic flow 

 there is developed in the Martin's Creek district an important type of rock, 

 somewhat tuffaceous, but not by any means a true tuft". It proves in thin 

 section to be a tuffaceous soda-rhyolite; it is pale gi-een in colour, albite pheno- 

 crysts being seen in hand-specimen. The rocks present features almost identical 

 with those of the group of similar rocks occurring at Currabubula, described by 

 W. R. Browne (Benson, Dun and Browne, 1920, Section C, p. 408). 



Just east of the road near Mt. Johnstone, close to the outcrop of these 

 sodic rocks, there is an interesting section of the underlying strata, down to the 

 next massive flow, which is a toscanite. Immediately following the toscanite is 

 an outcrop of the rocks described elsewhere as volcanic conglomerates; there is 

 abrupt variation along the strike, the rock in some places possessing all the 

 characters of the volcanic conglomerates in the Langlands Section, and else- 

 where being more of the breccia type which is predominant in the Glenoak 

 Section. Here this phase is very hard, consisting of cherty-looking fragments 

 and the usual secondary quartz which, in places, is itself replaced by crystals 

 of a red mineral having some of the properties of stilbite. The identity of this 



