RocLs iirdf Hcidhfotc. ;j35 



on the other hand, is typically plutonic in aspect, bein<r coarsely 

 crystalline and porphyritic. It is connected with the Cbbaw 

 massif which has metamorphosed tho Ordovicians in contact 

 Avith it. The Ordovieian shales near here are in places altered 

 to Chiastolite slate, and there can be little doubt but that the 

 coarse-trrained Lrranitic rock is responsible for the alteration, and 

 not the diabase or the latei" gi-anite-porphyi*y. 



B. AhiiormoJ Fedtnri's of tlic Didhase Series. 



(ci) Silicification. — The diabase of Tranter's paddock, Knows- 

 ley East, is in places chanjred to a dark, dense silicious rock, 

 somewhat reseniblinir tho black cherts, but showinsr no bedding 

 planes. 



South of the Gully at the south end of Red Hill there is a 

 southerly continuation of the foliated diabases, which here have 

 undergone remarkable chemical replacement. The rock presents 

 various stages in silicification, and in places solutions have 

 removed practically all the diabasic material, and there now re- 

 mains a very cavernous rock, consisting mainly of well-shaped 

 small quartz crystals. 



Priidiirtion of tliK J aspcroids. — The most interesting and 

 remarks Ijle of the metasoniatic changes is that to which the 

 formation of the red jasperoids is due. No discussion as to 

 the origin of rhese jasperoids has yet appeared. Dr. Howitt 

 and Professor Oregon- and Mr. Lidgey refer to them as meta- 

 morphic, and in a recent note (op. cit.) Mr. Dunn has described 

 their northward extension near Tooleen, where, he says, " they 

 are associated with an ancient igneous rock."" 



Jaspers appear always to be formed by metasomatic replace- 

 ment of certain rocks by silica, with the separation of some 

 oxide of iron. Red jaspers are associated with the ironstone of 

 Nowa Nowa, at the head of Lake Tyers, but in this case they 

 can be clearly seen to arise from the alteration of ancient, highly 

 folded sandstones. The jasperoids of Heathcote, however, are 

 derived from the diabase. The clearest evidence of their mode 

 of origin can be seen in a limited exposure of massive diabase, 

 about 40 yai-ds west of the Murray road, and about 300 yards 

 south of the selwynite outcrop. Here, within a few feet, all 



