Rocks of tJie Huivqua River. 41 



The Protospongia cherts bound the diabase along the eastern 



iside of the main mass, forming a well-defined belt, about half a 



.mile wide. Numerous specimens, showing the characteristic cruci- 

 form spicular arrangement of Protospongia, were found in the 



.field in these cherts, and a microscopic section. No. 78, from this 

 belt of cherts, displays well the abundance of the sponge spicules 



.embedded in a dark, largely chalcedonic ground mass. (See Micro- 



,photo Plate I., Fig. 3.) 



In the Heathcote area Dr. Howitt held that the silicification of 



• corresponding rocks was a metamorphic effect, due to the intrusion 



•of the diabase, an interpretation which is dissented from by Pro- 

 fessor Skeats, who shows good reasons for regarding them in part 



.as altered submarine tuffs, silicified by metasomatic action, the 

 adjacent igneous rock being considered as mainly contemporaneous 

 lava flows. 



The features are closely analogous here, where the Protospongia 



^cherts are found, lliese fossiliferous cherts have not been recog- 

 nised along the western boundary nor along the contact of the 



.smaller diabase occurrence. In general, the western junction of 

 the main mass is marked by much crushing and shearing, wit^i 



•other accompanying alterations in both the igneous and the sedi- 



.mentary, but nothing attributable to contact alteration has been 

 noted. Shistose talc rock is abundant in the valley of the Stock- 

 yard Creek, and elsewhere, close to the junction. An important 



.section is exposed in the bed of the Howqua at the foot bridge, 

 where the old dray track from Merrijig ends. The diabase crosses 

 the river in a north-west direction, just above the bridge, forming 

 a bar, and an interesting series of intensely crushed rocks of some- 

 what varied character can be traced more or less continuously from 

 within a few yards of the contact down stream for about three 



'Chains. A short break of about two chains intervenes, where the 

 exposure isi masked, and then crushed black slate follows, forming 



.a conspicuous cliff, at the bend of the river. The crush zone here 

 is at least five to six chains wide. A noteworthy feature is that the 

 rock adjoining the diabase shows not the slightest contact altera- 

 tion at a distance of less than a yard from the junction. In- 

 tensely crushed and contorted rocks are the general rule along the 



•contact. Ihis feature, together with lithological and fossil differ- 

 ences of this zone compared with that in the eastern edge are 

 worthy of note, and will call for further comment later, when 

 dealing with the sedimentary rocks and general structural con- 



rsiderations. 



