350 
BASALT AND OTHER IGNEOUS ROOKS ASSOCIATED WITH THE IPSWICH 
EORMATION. 
Tne relation of the basalt of Mount Tambourine to tbe Ipswicb T’orination is well 
shown by the remarhs made by Mr. Hands on the junction-line between the schists and 
Ipswich Formation. Tlie straight junction-line between the two formations can hardly 
bo anything else than a line of fault, and the basalt overlies not only the portion of the 
Ipswich Beds seen in that neighbourhood but also the schists and the still more recent 
fault. To the south and cast the basalt directly overlies the schists in the Maepherson 
Range, and in outlydng patches between that range and Burleigh Heads. 
In the Toowoomba Range the bedded basalt overlies a portion of the Ipswich 
Formation, and presents a succession of steep escarpments to the east. Outlying patches 
of basalt, overlying members of the Ipswich Formation, apparently on a lower horizon, 
form ‘ The Knobby ’ and other hills in the Rosewood Scrub. To the west of the Too- 
woomba Range, the basalt beds form no escarpments, but sink beneath an accumulation 
of strata, separated iiideed from the Ipswich Coal Measures of the other side of the range 
by an unconformabilitjr and an immense outburst of basaltic lavas, but still apparently 
belonging to the same series and characterised by the same fossils. To this western 
series belong the Coalfields of Jimbour and Clifton, which are, therefore, on a higher 
horizon than the Ipswic-h Coal Field proper. 
The basaltic lava-flows, therefore, were poured out after the deposition and 
partial upheaval of a groat part of the Ipswich Coal Measures, and were subsequently 
covered by a considerable thickness of strata still belonging to the period grouped, from 
lithological similarity and similarity of organic remains, as the IjDswich Formation. In 
short, they are contemporaneous with the Ipswich Formation. 
It is a mistake to suppose that the whole of the basalts comprised within the area 
occupied by the Ipswich Formation form one continuous series. In the neighbourhood 
of Ipswich, masses of basalt are clearly interbedded with the stratified rocks. {See 
Plate 49, fig. 1.) At Clifton and elsewhere masses of basalt appear to occupy local 
hollows in the stratified rocks, 
Mr. Rands describes the basalt of Tambourine Mountain as amygdaloidal on its 
upper surface, as including large patches of obsidian and pitchstone, as being “ generally 
full of olivine crystals,” and as being occasionally columnar, the hexagonal 2)illars being 
often twenty feet in length and three to four feet in diameter. 
Mr. Rands describes * a mass of trachytes containing beautifully developed 
crystals of sanidine felspar, as occurring in railway cuttings between Logan Village and 
Beaudesert, near Walton Station. It appears to have come up through the Ipswich 
Coal Measures, and to have flowed over a portion of them, as shown in the Section PI. 49, 
fig. 3. A similar rock is described as occurring about a mile west of Walton Station, 
apparently interbedded with the Ipswich Beds.” Mr. Rands classes these trachytes in 
his Albert and Logan Report as “ Older Volcanic” — i.e., older than the Toowoomba and 
Tambourine basalt and it appears tome on insufficient grounds. There can be little 
doubt, from the Geological Map attached to the Report, that the trachytes occur among 
strata, which are on a higher horizon in the Ipswich Formation than those on which the 
basalt of the Tambourine Mountain rests, and, whether interbedded or intrusive, they 
are not necessarily older, and may even be newer. 
Referring to the basalt, Mr. A. C. Gregory says f that “ the carbonaceous sand- 
stones and black shales are visible in the ravines within a quarter of a mile of the highest 
* Report on Logan and Albert, 1889. 
f Report on the Coal Deposits of the West Moreton and Darling Downs Districts. Brisbane: by 
Authority : 1876. 
