334 iV. li. Junner : 



.sco})i(.- ])ri!siiis of lutile or zircon, and linearly arranged gas and 

 liquid inclusions aie present in the quartz grains. Strain polarisa- 

 tion is a common feature in the quartz grains. Muscovite is i'airly 

 common, and occurs as twisted flakes which are frequently bordered 

 by green chlorite. Chlorite also intrudes itself along cleavage 

 planes. A brownish green variety of chlorite is more common, and 

 it api^ears to be an alteration product of some iron magnesium 

 mineral, as biotite. Biotite and an acid plagioclase also occur 

 in small amounts in nearly all sections. Of the usual concentrates 

 found in such rocks as these, zircon and touinialine are most 

 common. The detrital origin of these is evident by the rounding 

 of the salient angles in the prismatic crystals, llutile is not un- 

 common, and generally occurs in brown j^rismatic crystals. The 

 individual cpiartz grains are generally not very closely packed, 

 and the cementing material usually is micro- or cryptocrystalline 

 silica, which is often stained with brown hydrated iron oxide. 

 Occasionally tine sericitic mica and chloritic material form the 

 bond. In the fossiliferous g-rit from Warrandyte. the cement is 

 largely calcite. The nature of the rocks from which the silurian 

 sediments were derived has been discussed by Jutson.i He showed 

 that the pebbles in the Warrandyte conglomerate were practically 

 entirely of a sedimentary nature, i.e., quartz, quartzite and sand- 

 stone, and no jDebbles of an igneous rock were present, and this 

 led him to conclude that the rocks from which the conglomerates 

 were derived consisted largely, if not entirely, of altered and 

 unaltered sediments. In such an old conglomerate as this, 

 however, we might well expect to find onl}' the more resistant 

 rock types, like the ones above-mentioned, remaining, although 

 igneous rocks may have originally been present. On petrological 

 grounds, the writer draws the conclusion that the sandstones were 

 derived to a fair extent at least from a pre-silurian igneous rock, 

 probably granitic. Tliis view is supported by the following evi- 

 dence : — 



(1) The abundance of mu.scovite ; 



(2) The presence of biotite and plagioclase, and chlorite, which 

 is usually derived from unstable iron magnesium minerals; 



(3) The occurrence of zircon and rutile ci'vstals in the quartz 

 grains in the sandstone may indicate an igneous origin for such 

 quartz. 



(4) The constant presence of tourmaline sup])orts such an origin ; 



(5) The absence of metamorphic minerals, garnet, etc., show that 

 they were not derived from metamorphic rocks. 



1 p, p. .'■.32. 



