PREFACE. vii 



may exist, and, in consequence, have samples of rock pro- 

 perly assayed. It is unreasonable to expect an ounce reef 

 to show much free gold even on the outcrop, or by panning 

 out, especially if the gold is in a very finely-divided state. 

 Many years ago, I visited a very extensive gold mine in 

 New Zealand, and never saw a trace of gold in the immense 

 heaps of ore ready for crushing. In this mine the gold was 

 found in the free state and not much mixed with sulphides. 

 So, too, in one of the large mines of Johannesburg 

 a fifteen-pennyweights-to-the-ton mine the output from 

 which is more than 10,000 tons per month, the same 

 thing occurred, the gold being concealed, in a very fine 

 state, in the iron pyrites crystals. 



In connection with precious stones, mention has been 

 made of a small instrument which, so long as a certain 

 amount of experimenting has been previously made with 

 specimens, might be of much utility to prospectors who 

 usually know but very little about gem stones, and yet who 

 are very likely to meet with them in alluvial washings. 



Finally, I take the opportunity of reminding the pro- 

 spector who has to deal with surface rocks of a point of 

 great importance. Eocks and minerals have to be written 

 about, more or less, as if they were cabinet specimens, 

 although (as every one will understand) many of them have 

 been weathered for thousands of years. Even a description 

 of the appearance of an unweathered rock does not fix 

 itself in a student's mind so well as does the handling or 

 the examination of a specimen. For this reason, I should 

 advise any one who intends setting out on an exploration 

 to make himself as familiar as he can beforehand with the 

 appearance of the most important rocks such as granite, 

 diorite, schists, silurian rocks, &c., and to examine as 

 many gossans as he can, as well as all kinds of oxides, not 

 forgetting tinstone, in various colours ; carbonates, chlorides, 

 &c., of the various metals. After which he should learn 



