I S. P. EMMONS — THEORIES OF ORE DEPOSITION 



anything worthy of that name would far exceed the proper limits of an 

 address. Thus its scope has been gradually narrowed to -fit the neces- 

 sities of the occasion, until it has become little more than a brief enu- 

 meration of the opinions, held from time to time within the historic 

 period, which seem to have left the most permanent impress upon the 

 minds of geologists. 



The term " ore deposition," which is used in preference to its earlier 

 synonym " vein formation," as more correctly representing the broader 

 conceptions of the present day, applies, it is hardly necessary to state, 

 only to the processes involved in the formation of deposits that form an 

 integral part of the rock in which they occur, or " rock in place," as is the 

 legal phraseology of the day, and does not include such recent detrital 

 deposits as placers, etcetera, about whose origin there has never been 

 any wide divergence of opinion. 



Prehistorical Views 



The historic period is assumed to have been entered on only with the 

 revival of learning about the time of the Reformation at the commence- 

 ment of the sixteenth century. What few records can be found of genetic 

 opinions held before that time, even as to the more striking and readily 

 observable geological phenomena, such as volcanic eruptions, earth- 

 quakes, and changes in the earth's surface, are too scattered and frag- 

 mentary to afford evidence of any continuous development of thought. 

 The views of the Pythagorean and Aristotelian schools of philosophy 

 on the causes of these natural phenomena, though apparently based 

 more on bold poetical fantasy than exact observation, present a clearer 

 and more logical conception than that which obtained nearly twenty 

 centuries later. Thus, it is said that as early as Origenes, 600 B. C, the 

 observed occurrence in the rocks of casts of shells and plants were 

 ascribed to periodical floodings of the land. During the Middle ages, 

 however, under the monkish influence that discouraged any views that 

 might throw doubt on the literal correctness of the Mosaic cosmogony 3 

 these fossils were variously assumed to have been formed in place by 

 the agencies of the stars, to have been transformed from rock by some 

 plastic force (vis plastica) , or left by the waters of the Noachian deluge. 



Among the early cosmogonies, which it is true are of mythologic 

 rather than of scientific interest, the Chinese is the only one which in- 

 cluded metal among the elements of creation. Yet the general use of 

 the metals, whose extraction from their native ores presupposes a knowl- 

 edge of the art of smelting — in itself an evidence of a certain insight into 

 nature's processes — goes back to very remote antiquity. It seems possi- 



