Rocks of the Howqua River. 43- 



Class M.'- = ^^ < ? > - = Class 2. Dosalane. 

 Fern. 31.56^1 3 



O 72 1 



Order ^' = — ^ — < - = Order 5. Geimanare. 

 Fern. 64.45 7 



Rang. ^^O + Na^^m 5 3^jj ^^jj^^j. Calcic. Ilmenose. 

 ^ CaO 120 3 5 ^ 



Sub-Eantr. = _-2 — = — <_ > = Siib-Eang. = Persodic. Beerbacbose. 



(b) The petrography of the diabases ^ including soda-rich 



types. 



(By Professor E. W. Skeats.) 



Reference to tlie chemical analyses shows that diabases of fairly 

 normal composition are present. Ihe analysis of No. 15 shoAvs 

 that as regards the alkali content, it is normal, but has a high 

 content of the alkaline earths, especially of lime. No. 25, however, 

 on analysis proves to have a very high soda content, and is un- 

 usually low in lime. It evidently has suffered albitization, and 

 shows that the spilite type of magma is represented among the 

 Howqua diabases, and the analysis of a diabase from Heathcote is • 

 also relatively rich in soda. 



The petrographic descriptions which follow show that albitiza- 

 tion, silicification, chloritization, the production of secondary horn- 

 blendes from augites, the formation of carbonates, and the develop- 

 ment of the mineral lawsonite, are features of fairly common oc- 

 currence among the Howqua diabases, which in hand specimen,, 

 m.ay not appear to be specially altered. 



The term diabase, used to describe these rocks, has been found 

 useful to retain, although it is now clear from their field relations- 

 and principal petrographic characters that they are mainly subma- 

 rine lava flows and ashes. 



No. 11. — Diabase from the ridge between Howqua River and 

 Lick Hole Creek. The rock consists of pale-coloured augite to the- 

 intent of about 3-5th, and a very kaolinized felspar to the extent 

 of about 2-5th of its volume. The augite is diallagic in part. Both 

 augite and felspar seem to have started crystallizing simultaneously, 

 as each in places is porphyritic, and in other places is moulded on 

 the other. The felspar is too cloudy for specific determination, a 

 little ilmenite, altered to leucoxene and of chlorite after augite, is 

 also present. Some secondary clear to cloudy chalcedony is also- 

 present, partly replacing felspar. 



