1906.] on Ore Deposits and their Distribution in Depth. 81:^ 



in scattered patches ; and it is only where they have been formed on 

 the coohng margins of the rock, that we have much chie to their 

 distribution. But these ores are of secondary importance ; 1)ecause 

 iron ores do not pav to mine at great depths ; and it is only for ii*on 

 ores that this igneous origin is conclusively established. There is no 

 equal reason for regarding ores of such metals as copper, lead, zinc, 

 gold, and silver as derived from igneous rocks. 



The arguments for regarding igneous rocks as the source of ores 

 in general, are their constant companionship and the frequent occur- 

 rence of traces of various metals in igneous rocks. Neither fact seems 

 to me convincing. The frequent association of igneous rocks and ores 

 is natural, for the intrusion of the igneous rocks supphes the heat, 

 and makes the fractures necessary for ore formation. In regard to 

 the second argument, it is true that minute traces of various metals 

 have been detected in many igneous rocks, but usually in those of 

 mining fields. It is very doubtful whether the metals are primary 

 constituents. It is far more probable that they have been introduced 

 after the consolidation of the rock, and at the time of the formation 

 of the associated lodes. 



The Barysphere as the Source of Ores. 



For most ores, we are therefore driven back to the third possible 

 source— the hidden interior of the earth, which is called the centre- 

 sphere from its position, and the barysphere on account of the weight 

 of its constituents. This is an ample source of metals, for we may 

 regard the earth as a vast ball of iron, hardened, like a modern 

 projectile, by nickel, and no doubt charged with other metals. The 

 rocks of the earth's crust are the stony slag given off from the cooling 

 metallic mass. The one objection to this source is the great depth of 

 the metallic barysphere ; the specific gravity of the rocks at the sur- 

 face is about 2-5, and that of the earth as a whole about 5'0. At 

 the depth of 100 miles, the specific gravity of the rocks is estimated 

 to be a])out 2*8, or merely that of the basic igneous rocks. But in 

 all proljability the surface of the barysphere is not regular ; projec- 

 tions from it rise into the lithosphere, and the mining fields probably 

 lie over these projections. 



There are areas, such as the Scotch Highlands, traversed by faults 

 of colossal size, and with the rocks intensely metamorphosed and 

 charged with quartz veins, but these veins are not, to use the usual 

 phrase, mineralised ; there are enough quartz veins to maintain a 

 large mining field ; but, with insignificant exceptions, they are barren. 

 The barysphere may be so far below, that the quartz veins have not 

 been charged with metals ; whereas in other places, owing to the 

 barysphere being nearer the surface, the veins received metallic, as 

 well as non-metallic, minerals. 



