Rocks of the Hotvqua River. 49 



It can be matched exactly with a similar occurrence at Heathcote 

 south of Photograph Knob, where Professor Skeats regards it as 

 an altered tuff. 



B. Agglomerate. 



A well-marked breccia occurs on the east- west ridge between the 

 Howqua and Fry's, and the Lick Hole Creek. It forms one of the 

 pronniiLxit points, and is marked Breccia Knob on the map. 

 This rock is a typical breccia, made up largely of igneous frag- 

 ments, but contains also some banded chert. Sections of the chert 

 examined suggested strongly altered tuff, and were of coarser texture 

 than the Protospongia chert. Under the microscope a thin section 

 of the breccia (Section 14) shows it to be made of fragments so 

 dense as to be almost opague, but small pyroxenes present indicate 

 that the material is igneous and represents rapidly cooled lava. 

 If the eruption were submarine, as it is believed the general evi- 

 dence indicates, the rapid chilling of parts of the lava would be 

 expected. 



C. Alteration Features of the Diabase, 

 (a) The Red Jasper. 



Tbe bright red colour of this rock together, with its hardness 

 and durability make it rather a conspicuous and characteristc rock 

 in the recent river gravels in the diabase region, and also an easily 

 recognizable pebble in some of the Upper Palaeozoic conglomerates. 



It occurs in situ in the diabase as apparent inclusions, ^ for which 

 they have have been mistaken, but it is clear in the Howqua, as 

 in the Heathcote region, as shown by Professor Skeats, that it 

 represents one of the phases of metasomatic replacement of the 

 diabase. A good' section can be studied in the bed of the Howqua 

 River at low water, about one and a-half miles above Fry's, as. 

 shown on the map. The irregular shape is typical of patchy re- 

 placement areas, and miscroscopic sections occasionally show relic 

 structures of the original igneous rocks, though in general the 

 action has gone so far that all that is seen is an aggregate of 

 secondary quartz and iron oxide stain. 



These jaspers occur at intervals along a definite line, bearing 

 from north-west to south-east, that is, coinciding with the genera! 

 trend of the diabase. 



They vary from small aggregates of jasper patches to larger 

 masses of perhaps 50 to 100 square yards in extent. Associated 

 with the normal red jasper there are often other varieties of quartz 

 varying both in colour and texture, milky quartz and granular 

 quartzitic forms being frequently present. 



