TRESIDEXTIAL ADDRESS SECTION B. 49 



upper part of au ore-Lodj', and lieuce the probabilities of the 

 downward extension of the valuable mineral. This is clearly 

 by far the most important matter in estimating- the possi- 

 bilities of newly-discovered lodes or mineral districts, and one 

 to which far too little attention has been paid in the past, 

 both in technical literature and in mining education. Newly- 

 opened properties are frequently reported on by men who may 

 be experienced in running- mines, but have little or no con- 

 ception of the influence of surface enrichment processes. Yet 

 comparatively undeveloped deposits require for a reasonably 

 correct appraisement of their possibilities an examination 

 made with every assistance that the scientific study of ore- 

 bodies can afford. Their investigation comes, in fact, Avithin 

 the province of the mining geologist rather than that of the 

 mining engineer. At the same time, the raw geologist may 

 be as dangerous a guide in certain cases as the so-called 

 " practical man," because it is absolutely necessary in solving 

 the problems presented by mineral lodes to have due regard 

 to those severely practical considerations Avhich may be 

 summed up under the head of working facilities. 



There are, of course, instances in which secondary 

 enrichment processes may render a lode workable to a con- 

 siderable depth, even when the primary ore is altogether 

 unprofitable. In the case of such metals as iron, the masses 

 of ore produced in the course of surface enrichment are often 

 so large that they constitute by far the most considerable 

 of the deposits which are commercially exjiloited. Then, too, 

 in dealing with metals like copper, zinc and lead, the possi- 

 bility of impoverishment of the outcrops, or even the removal 

 by leaching of much of the metal-bearing mineral at various 

 other points, especially in the neighbourhood of water-level, 

 must be borne in mind. An iron-bearing gossan may be all that 

 represents a rich copper lode on the surface. We have also 

 to consider what may be termed " mixed deposits," such as 

 copper-tin lodes or lead-zinc lodes, in M'hicli one metal tends 

 to replace the other in depth. Thus many of the Cornish tin 

 mines began their career as producers of copper, while the 

 great Broken Hill lode in Australia, and that of the same 

 name in Northern Rhodesia, commenced as producers of lead, 

 but are turning into zinc mines as they are followed downward. 

 The problems presented by such deposits are of great com- 

 plexity, and experience alone can teach the observer how to 

 avoid the manj' pitfalls which beset his path. 



Turning to another side of the science of ore deposits, 

 we come to the elucidation of the blanks and breaks which 

 may occur in the distribution of the valuable minerals through 

 a lode. In many lodes a particular section is often much 

 the richest, and may alone be 73rofitable — for instance, the 

 footwall may be very rich and the rest so poor as scarcely 

 to be worth mining*. Still more frequently the valuable 

 portions form distinct patches, M'hich may be sufficiently well 

 defined to constitute what are commonly known as shoots 

 of ore. Then, again, we may have another kind of dis- 

 continuity, due to the action of such geological processes as 



