234 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Nov. 4, 



3rd. White quartzose drift and friable sandstones. 



The older basalt is usually much decomposed, and contains large 

 and beautiful specimens of semi-opal {hydrate of silica). 



The newer or post-tertiary basalt is quarried extensively for building- 

 purposes. It is of a dark bluish-grey colour, and, when broken, rarely 

 shows any signs of decomposition. It is in some places covered with 

 a quartz- drift and fine sand, of the same age, most probably, as the 

 newer auriferous drift which occurs generally on the gold-fields*. 



On the basaltic plains are found what are locally termed " dead 

 men's graves." These are low narrow hillocks, lying close together, 

 and really not very unlike the mounds of a cemetery. They are 

 formed, I believe, in the following manner : — The basalt is split up 

 into large blocks by the shrinkage-cracks, and, as the edges of the 

 blocks decompose, they come to have a rounded form, the superficial 

 hillocks answering to each block, and the depressions to the shrinkage- 

 cracks. This explanation presented itself after an examination of 

 some quarries in such localities. 



The area occupied by recent basalts is, at a low computation, 

 three thousand five hundred square miles. Much of the country 

 is very fine agricultural land, and some of the ancient craters are 

 well grassed and beautifully timbered. The flat plains are gene- 

 rally arid in summer and swampy in winter, but are used profitably 

 for feeding cattle and sheep, and, under good management, produce 

 excellent crops of oaten hay. 



From a careful inspection of a large extent of country covered by 

 basaltic rocks, and from information collected from various sources f , 

 I incline to the opinion — 



1st. That the newer basalt was erupted at a period when consi- 

 derable areas, both north and south of the main coast-range, were 

 submerged ; but the appearance of much of the basalt shows that 

 it must have cooled rapidly, and not under very great pressure. 



2nd. These eruptions do not appear to have disturbed the " mio- 

 cene" tertiary beds, which are generally found nearly horizontal. 



3rd. Tli at, from the occurrence near many extinct volcanos of 

 porous lava, vesicular basalt, and obsidian J associated with frag- 

 ments of white pumice, it is probable that, subsequently to the depo- 

 sition of the latest tertiary drift (pleistocene) which overlies the 



* See Geological Surveyor's Third Report. 



t More especially I am indebted to Mr. A. R. C. Selwyn, the Government 

 Geologist, whose excellent maps, now embracing the whole of the county of 

 Mornington, a large portion of the counties of Evelyn, Bourke, Dalhousie, and 

 Talbot, and the unsettled districts north of Talbot, are now available ; and, though 

 they do not extend to the interesting volcanic district west of the River Werribee, 

 they embrace the chain of extinct volcanos from the River Plenty to Mount Aitkin. 

 In addition to allowing access to those maps (in themselves most valuable), Mr. 

 Selwyn is at all times willing to afford the fullest information, and freely commu- 

 nicates facts ascertained during his explorations, even before he has had time to 

 place them on record in his official reports. 



X Fine specimens of obsidian, with small fragments of white pumice adhering 

 to it, are found at Geelong. The best specimens in my collection were collected 

 and sent to me by A. J. Skeine, Esq., District-Surveyor, &c. 



