314 Professor John W. Gregory [April '11, 



The Traxsportixg Agents. 



Accepting the barysphere as the source of metals, we have to 

 consider the possible agencies by which they have been raised to their 

 present level. 



Iron has been brought to the surface of the earth by the intrusion 

 of igneous rocks from the deeper parts of the lithosphere ; and some 

 quartz veins are also of igneous origin, having been formed as intru- 

 sions of molten rock. But these igneous quartz veins are barren, 

 probably because they were too hot to be a suitable medium for the 

 deposition of metiils. 



The great majority of ores have been formed by percolating water, 

 as is shown by their arrangement in mass, by their microscopic 

 structure, and by the minerals associated with them. 



The introduction of ores in solution is indicated by what is known 

 as " crustifi cation " — their deposition in layers along the two walls of 

 an ore-filled fissure ; also by the frequent occurrence of geodes — cavi- 

 ties filled by the slow infiltration of material through cracks ; also by 

 the association of ores with such minerals as calcite, which are of 

 aqueous origin, and the rarity of the typical minerals of igneous 

 rocks, such as the felspars and the pyroxenes ; and when lodes contain 

 minerals, such as quartz or mica, common to both igneous and sedi- 

 mentary rocks, the varieties present are those deposited from solution 

 or formed by the re-crystallisation of materials by crushing along 

 fractures. Ores, of course, occur in igneous rocks ; but in such cases 

 they have usually been introduced in solution, and have replaced the 

 original minerals. 



The Descent of Meteoric AVater. 



The formation of most ores by water is now universally accepted, 

 so their distribution in depth is controlled by the distribution of 

 underground water. There are two sources of subterranean water. 

 The first is the rain, which supplies the meteoric water. Some of this 

 water runs off in rivers to the sea ; another part of it, sometimes the 

 whole, is restored to the air l)y evaporation, and the rest of it disap- 

 pears by percolation underground. 



The amount of this percolation seems to me to have been greatly 

 exaggerated in the past. The amount lost by evaporation has ])een 

 underestimated, and the rocks have, therefore, been regarded as far 

 more permeable than they really appear to be. This exaggerated 

 estimate of the descent of the surface waters has been due in the 

 main to three ideas : — 



1. An underestimate of the amount of water available from other 

 sources for the supply of deep wells. It was necessary to believe that 

 a large percentage of the rain must percolate underground, and that 

 rocks are very permcMble, so long as the deep artesian vy-ells of arid 



