The Hodghinson Goldjield, Northern Queensland. 9 



hornblendic fragments in the dykes seemed to favour, as they 

 are not unlike fragmentary plant impressions. No positive 

 assertion can be made as to the age of these slates ; but 

 their resemblance to the Victorian auriferous strata, their 

 lithological character and vertical dip, render it extremely 

 probable that they are of lower silurian age. The paral- 

 lelism of the dip of the strata, or rather folds, and the 

 general direction of the ranges, render it probable that 

 we have in the physical features of this country the few 

 remaining relics of the corrugations or folds into which the 

 earth's crust shrank subsequent to the lower silurian period. 

 But we have evidence that there have been immense changes 

 since. In the accompanying figure of Mount Mulligan, 

 about twelve miles from Thornboro, we have another relic, 

 but of a different kind, of the former geological history of 

 the country. This is an isolated mass of tableland, the 

 summit of which is composed of about 600 feet of horizontal 

 sandstone, exactly like the Hawkesbury sandstone, as it is 

 seen at the Port Jackson Heads or along the railway line 

 over the Blue Mountains. From the occurrence of the 

 formation at Mount Mulligan and numerous other places on 

 the coast, there can be but little doubt that this horizontal 

 sandstone once covered immense tracts of north-eastern 

 Australia ; but it is only seen now in isolated patches, all 

 the rest having been completely denuded away. I cannot 

 offer any direct evidence that the Mount Mulligan sand- 

 stones are the equivalents of the Hawkesbury sandstone, 

 because I was not able to examine them closely; but 

 lithologically there is no difference, and similar outlying 

 patches are known to contain vegetable impressions, con- 

 necting them with the formation in New South Wales. It 

 will be seen by the sketch annexed that the sandstone rests 

 unconformably upon the upright edges of the vertical slates. 

 These slates are full of quartz reefs, some of which are 

 worked for gold. 



I would here call attention to the large number of 

 inferences which can be drawn from the Mount Mulligan 

 formation. It is obvious that the silurian rocks were not 

 only deposited but contorted so as to be tilted to then- 

 present vertical dip before the sandstones were deposited 

 upon them. The quartz reefs were also formed on them. 

 But we know from the geological record elsewhere that an 

 immense number of formations and a number of distinct 

 epochs intervened between the slates and the age of the 



