president's address SECTIOX c. 195 



The Diabases or Andesites of the Buchax District. 

 Geographical and Geological Relationships. 



In two or three localities in the Buchan district, notably at 

 Murendal, at Back Creek, and at Moore's Crossing of the Snowy- 

 River, the felsitic fragmental rocks just described are directly 

 succeeded not by the Buchan limestone but by a more basic series of 

 igneous rocks. Tliese have been described by Howitt (51) as diabases 

 and diabase-porphyrites. 



At Murendal these rocks are intrusive into the Lower Buchan 

 felsitic series. At the Back Creek and at Moore's Crossing of the 

 Snowy River they underlie the calcareous Upper Buchan limestones 

 with apparently passage beds between the two series. At Murendal 

 they appear to be dykes, while in the other localities they occvir 

 as lava flows. Their stratigraphical relations are clearly fixed a„s 

 coming between the Lower Buchan felsitic rocks below and the 

 Mid-Devonian Buchan limestones above. 



Petrological and Chemical Characters. 



Most of the rocks are free from olivine, but in the adit of the 

 Murendal South Mine olivine bearing basic rocks occur which he 

 groups with the other basic rocks of the Buchan district, but also 

 compares with the Upper Devonian or L. Cai'bonifei'ous diabase of 

 the Snowy Bluff. 



At the Murendal mine the diabase contains tabular plagioclase, 

 colourless augite, and magnetite as the most abundant phenocrysts 

 in the order mentioned, while minute plagioclase prisms, magnetite 

 and apatite needles occur in the gi'ound mass, which has, in addition, 

 a little pale yellow to colourless glassy base. 



An analysis of this rock by Howitt is as follows : — 



SiOo 4S-4S K„0 1-77 



Al,6, 14-57 Na„0 3-33 



Fe„05 11-68 H„0 1-72 



FeO 2-83 



CaO 9-56 



MgO 5-55 



FeO 2-83 CO., 1-27 



CaO 9-56 Pods -45 



Total 101-21 



A rock from Moore's Crossing, Snowy River, is described by 

 Howitt as a diabase-porphyi'ite, phenocrysts of plagioclase, diopside, 

 enstatite altered to bastite and magnetite were present, while the 

 ground mass consisted of minute plagioclase prisms, magnetite, and a 

 little glassy base. 



The analysis of this rock is as follows : — 



SiO'o 53-39 K„0 1-84 



AUO3 15-23 Na„0 3-GO 



FejOa 8-73 H06 114 



FeO 3.(51 C6„ 0-22 



CaO 8-46 Pods 0-16 



MgO 4-12 



Total 100-50 



