The Tri'polite Deposits of Lilicur. 253 



The topographical features at the close of the Pliocene 

 period appear to have been, in many respects, analogous to 

 those just described. The lava of Lilicur and Caralulup hay 

 been, in part at least, derived from Mount Mitchell whence 

 it descended, and by successive sheets filled a then existing 

 river valley, the lower portion of which is now known to 

 us as the Homehush auriferous lead. During and subse- 

 quent to these lava flows the surface of the country was in 

 places pitted by shallow basins, which became harbours of 

 bacillaria and receptacles for their frustules. Owing to the 

 trifling extent and gentle slope of the area drained by the 

 individual basin, the effect of erosion was exceedingly 

 slight, and the amount of foreign sediment that was washed 

 into the basin so minute as to be imperceptible. Neverthe- 

 less, in view of the eminently ferruginous character of the 

 surface rock, the absence of ochreous matter in the body of 

 the tripolite is noteworthy. The circumstance explains 

 itself when it is borne in mind that a reducing agent in the 

 form of humic acid was unquestionably produced by the 

 decay of the organic parts of the diatoms themselves, and 

 that this production was probably sufficiently copious to 

 prevent, without the aid of other agencies, the settlement of 

 any ferric oxide that may have entered the pool. 



The principal tripolite bed in the district, as far as it has 

 been explored, is that situate on Mr. M'Kenzie's farm, on 

 allotment 15, section I, parish of Lilicur. The deposit com- 

 pletely fills a basin-shaped hollow in the basalt, the maxi- 

 mum depth being 17J feet. The surface is level, and the 

 contour, somewhat circular, with a mean diameter of about 

 500 feet, encloses an area of, approximately, 4| acres. The 

 quantity of tripolite in this basin (calculated from the above 

 dimensions) is 1^ million cube feet, which, taking the specific 

 gravity at 1*91, represents a bulk of 66,500 tons. A shaft 

 has been sunk near the centre of the deposit on to the bed 

 rock (a highly vesicular basalt, in places reduced to a warke), 

 and from it galleries have been driven in various directions 

 along the basalt floor, which is remarkably even, and has 

 a slight outward rise. The deposit itself has a tolerably 

 horizontal — albeit somewhat waviform — stratification, and 

 although strictly homogeneous in its structure, and of 

 uniform snowy whiteness, the laminse (more correctly speak- 

 ing, the layers of growth, varying from half an inch to three 

 inches in thickness, and extending from floor to roof) can be 

 traced for many yards along the walls of the galleries and to 



