27 



talcose, or cliloritic, according to tlie prevalence of the chief 

 ingredient. No fossils can be seen in these slates even under 

 the microscope, but their age has been proved to be lower 

 Paloeozoic by associated fossiliferous sandstones and lime- 

 stones. It is of great importance to note the age of the 

 sedimentary strata ; for in this continent the richest gold- 

 fields are found to be those whose out-cropping rocks are of 

 Silurian age, with intrusive dykes of granite and greenstone. 

 But gold-bearing rocks are not always necessarily confined to 

 one particular age, as is evidenced in America, where 

 auriferous quartz has been proved to be associated with later 

 formations than the Silurian and Devonian, 



Gold has evidently been deposited in common with other 

 metals throughout all ages, even up to the present time. In 

 Australia, though we find it associated only with the past, 

 its deposition was co-existent with the dawn of creation of 

 animal life. It belongs to the age of Invertebrata, when the 

 inhabitants of the seas consisted only of echinoderms, trilo- 

 bites, bivalves, cephalopods, etc. 



It was also the first great change of type in animal life, 

 when endo-skeletal creatures were sucaeeding the exo-skeletal 

 in the order of things and process of evolution, for the 

 remains of the earliest fishes are found in the uj^per beds of 

 the Silurian formation. 



It was at this time, then, that during active igneous eruptions 

 and intrusions gold and other minerals were brought into 

 association with the slialy sedimentary strata of the Silurian 

 and Lower Devonian systems. 



Gold has also been found in sedimentary rocks belonging 

 to the carboniferous formation : but no doubt in these cases 

 the gold has been derived from the original auriferous rocks 

 of the older systems. Its presence here, though, teaches us 

 that it is not impossible to find it in any formation, especially 

 when that formation is derived from the denudation of 

 Silurian strata. 



There is one feature about these auriferous formations that 

 is characteristic ; they are more or less crystalline or me- 

 tamorphosed, probably from the action of heat under great 

 pressure. But it is found that the less crystallised they are, 

 the more auriferous they appear. 



As regards the matrix of gold, quartz is almost universally 

 the most prevalent. And wherever it occurs it is always in 

 veins or reefs, occupying fissures and rents, either in the 

 primary rocks, or in the plutonic intrusive dykes. 



But gold is also found in other rocks besides quartz. Horn- 

 blendic granites and diorites, or a greenstone composed of 

 hornblende and feldspar, are most commonly the source of 

 the metal. Serpentine, too, has been worked profitably, and 



