THE VICTORIAN NATURALIST. 173 



however, is a very uncertain mineral, and its tendency to appear 

 and disappear unexpectedly is likely to have a depressing effect 

 on speculators in the mines of Arnhem's Land. There are in 

 many places along the edge of the granite country " pockets " of 

 extraordinary richness. They are not, I believe, connected with 

 veins. A little stream-tin has been found in a few streams, but 

 the great mass of the tin ore remains in its matrix in the 

 crystalline schists. 



Copper is found in many parts of the territory, and would be 

 found in many more if prospected for. Iron occurs everywhere, 

 and there are several indications of nickel, graphite, manganese, 

 zinc, antimony, and bismuth. I have also seen excellent 

 specimens of platinum, which is said to be abundant. 



This nearly completes all that is to be said about the economic 

 geology of Arnhem's Land. There is, however, something 

 remaining to be told about curious and interesting geological 

 facts. First, with regard to the volcanic period, which I consider 

 to be intimately connected with the desert sandstone. The 

 consequences of the pliocene volcanic period are of tremendous 

 importance. The line of volcanic action seems to have kept 

 mostly along the sea coast, nearly all round the continent, and 

 this fact would indicate that it had something to do either with 

 subsidence or upheaval. The most extensive evidences of 

 upheaval are on the south side, where it has taken place to the 

 extent of about 600 ft. for over 1,000 miles. A tremendous 

 ejection of material from below, chiefly volcanic ash, dust, glass, 

 and sand, with lesser quantities of lava, Szc, has accompanied 

 the volcanic period, covered the ground with hills of sandstone, 

 and rendered the sea of North Australia shallow for a great 

 distance off the coast. The chemical character of the ash strata 

 was very varied, as they were composed of ferruginous and 

 siliceous sandstones, magnesite and volcanic glass, and had 

 many other local peculiarities. 



The physical features of these volcanic areas are most 

 interesting. Having fallen, like snow, in loose heaps not more 

 than 700 ft. thick, they only lay undisturbed where the foundation 

 was secure. It will be easily understood that in the river valleys 

 they would be swept aside, and form cliffs overhanging the water. 

 These valleys are now lined with great boulders and piles of 

 sandstone, looking as if tremendous earthquakes had shaken and 

 riven the rocks to their foundation. But, in truth, there have 

 been no subterranean forces, and the present state of the strata 

 is probably due to some portions having become sufficiently 

 hardened to resist the action of the water, while the looser 

 portions have been washed away. The effect of running water 

 on loose volcanic beds is clearly shown in the cliffs and gorges 

 of several of the rivers in the Territory. Doubtless these same 



