president's address SECTION c. 201 



of various age, but all oltler than the Kainozoic, and no rocks rest upon 

 them. In some cases, such as those of French and Philip Islands, the 

 proximity of proved Lower Kainozoic basalt at Flinders, together with 

 the general field relations, suffice to make it probable that the age of 

 the basalt in all three areas is the same. Many of the occurrences 

 are separated by considerable distances from basalts whose age can 

 bo demonstrated, and in these cases considerable doubt remains as to 

 whether they are correctly referred to the older basalt, and if so, as 

 to whether they are contemporaneous or part of a series of eruptions 

 e.xtending over a long period of time. 



Petrological and Chemical Characters. 



It is frequently stated that the older basalts can be distinguished 

 from the newer basalts by two characters — i.e., their finergrained 

 dense texture and the greater alteration they have suffered. This 

 is sometimes the case, but the more I have seen of the Victorian 

 basalts in the field and under the microscope the less reliance I feel 

 inclined to place on these generalisations. I know of all gradations 

 from a coarse dolerite to a glassy tachylyte among the newer basalts 

 and also from among those areas classed as older basalt. Fresh 

 and decomposed basalts occur among each series. Neither in 

 hand specimens nor in section do the Victorian basalts present 

 petrological characters sufficiently distinctive to enable me to say 

 whether a given specimen comes from the Older or the Newer series. 



Short descriptions of a few sections from different localities will 

 serve to indicate the variety of types included within the older 

 basalts. 



Howitt has described (102) a considerable number of the basalts 

 in North and East Gippsland. 



A Tachylyte (or Basalt vitrophyre) is described by Howitt from 

 the Tangil Deep Leads Mine (112). The rock is a mixture of volcanic 

 glass and fragments of crystalline rock. The glass is black in hand 

 specimen and yellow and isotropic under the microscope, with small 

 lath-shaped labradorite, colourless olivine, magnetite and chlorite and 

 carbonates as alteration products. The sp. gr. of the rock is 2'61, 

 and the analysis given below shows that the rock is a tachylyte. 



Another Tachylyte collected by Howitt from Butcher's Creek, 

 Gelantipy, shows in section numerous minute perfectly formed crystals 

 of plagioclase and colourless olivine crystals. The bulk of the rock 

 consists of a light-brown isotropic glass in which the crystals are 

 embedded. 



A dense older basalt from Grice's Creek, between Frankston and 

 Mornington, has a small amount of a glassy base, granular augite and 

 magnetite, porphyritic crystals of olivine and abundant small 

 elongated lath-shaped plagioclase with some indications of flow struc- 

 ture. Another dense rock from Leongatha differs from the above in 

 the very fine granular texture of the rock. Lath-shaped plagioclase is 

 only sparingly represented, minute granular augites and magnetite 

 with some olivine making up the bulk of the rock. 



