Ordovician Sediments. f)3 



5. Origin of the Mineral Contents. 



W. G. Langford,^ in his discussion of the constitution and 

 origin of the Melbourne Silurian Sediments, pointed out that 

 there were two possible sources of the material for the silurian 

 sediments. So also there are two possible sources of the Ordo- 

 vician sediments : — 



(a) They may have been derived from a pre-Ordovi- 

 cian igneous rock.. 



(b) They may have been derived from a pre-Ordovician 

 sedimentary rock. 



The presence of such minerals as muscovite, tourmaline, zir- 

 con, rutile, ilmenite, magnetite, apatite, topaz, and sphene would 

 perhaps point to an igneous rock as the origin of the sediments, 

 but being stable minerals they may easily undergo transporta- 

 tion from the sediments of one period to a later. 



Biotite is very rare, and is generally altered to chlorite, but 

 its presence would indicate either an igneous or a metamorphic 

 origin, as would also the fresh felspars which are occasionally 

 met with. In the sandstones, felspar grains are very few in 

 number compared with the quartz, and are sometimes repre- 

 sented by turbid patches. The slates and the fine sericitic 

 ground-mass of the sandstones, however, are purely argillaceous, 

 and must have originally been of the nature of clay, which in 

 its turn, must have come into being through the prolonged 

 breakdown of felspars. The extreme fineness of this clayey 

 material would rather point to an older sediment as the deriva- 

 tion of the greater part of it. The clear unaltered felspars, 

 though few in number, would tend to show that at least part 

 of the constituent mineral content was derived from an old ig- 

 neous or metamorphic rock. 



The gradual transition from Heathcotian to Lower Ordovi- 

 cian throughout Victoria eliminates the possibility of the Heath- 

 cotian being the source of the material, whilst any possible Pre- 

 Cambrian outcrops are quite unknown anywhere within 100 

 miles of Bendigo. 



The writer pictures, then, in the Lower Ordovician period, 

 a gradually sinking landmass, probably to the east, over which 

 outcropped Pre-Cambrian metamorphic sediments, intruded per- 

 haps by occasional igneous masses. The denudation of this land 



6. "The Petrology of the Silurian Sediments near Melbourne." W. G. liangford, B.Sc, Proc. 

 Roy. Soc. Vic, Vol. XXIX., n.s., Part I., 1916. 



