310 Professor John W. Gregory [April 27, 



them as primary constituents ; they appear to have been subsequently 

 introduced, for they are associated with minerals, which the micro- 

 scope proves to be secondary in origin. 



The study of ores in recent years has made great progress by 

 recognition of the fact that an ore is a rock, and must be studied on 

 the same lines as ordinary rocks. The microscopic study of ores is 

 throwing much the same light on their depth and distribution, as it 

 has done on the depth and genesis of igneous rocks. A rock is 

 investigated both in the field and under the microscope in reference 

 to three main problems : first, the origin of the materials of which 

 the rock is composed ; second, the nature of the agents which have 

 carried the materials to their present position ; and third, what 

 deposited them there. 



The materials of any rock or ore can only come from one of 

 three sources : — 



1. Matter dissolved in the waters of the sea. 



2. The igneous rocks which, either directly, or indirectly as the 

 sedimentary rocks derived from their destruction, form the whole of 

 the earth's crust. 



3. The interior of the earth. 



Sea Water AvS a Source of (tOld. 



Gold is the only metal for which the sea is considered a possible 

 original source. The existence of gold in sea water was first seriously 

 advocated about fifty years ago to explain the occurrence of gold in 

 some plates of Muntz metal that had been used as the sheathing of a 

 ship, which had been trading for three years in the Pacific. It is 

 true that plates of Muntz metal fresh from the works also contained a 

 trace of gold. But the amount in the plates that had been exposed 

 to the sea was larger than in new plates, and the excess was explained 

 as due to gold electrolytically deposited from sea water. This view 

 appeared the more plausible when, in 1872, Sonstedt announced that 

 ne had detected the presence of gold in minute quantities in the 

 water of the Irish Sea. Professor Liversidge of Sydney, who has 

 followed up this question with greater perseverance than any other 

 chemist of equal distinction, has detected from | to 1| grains of gold 

 to the ton of sea water from samples collected on the coasts of 

 Australia, and in the middle of the Indian and Pacific Oceans. 



A grain of gold dissolved in a ton of sea water is not a very large 

 amount. One grain per ton is about 0* 0000006 per cent. And 

 condensed sea-salts, even if none of the gold were lost in evaporation, 

 would have but one grain of gold in 7S lb. of sea-salt, or one part 

 in 546,000. To detect these amounts requires chemicals of w^onderful 

 purity and chemists of the highest skill. It is not, therefore, sur- 

 prising that some chemists have failed to find gold in the sea, or 



