'rHEORII<:S OF ORE DTSPOSITTON HTSTORTCALLY CONSIDERED. 811 



oven attonipt a description of the mode of oeeiirrence of the ores, 

 much less specuhite on their origin. 



The historic time he-re contem phi ted may he divided, in a general 

 wav, into three periods, according to the ])i-evailing metliod l)y Avhich 

 the views th(Mi current were ari'i\'ed at. 



»EVP:L()P.AIEN'r OF KNOWEEUCiE II ISTORIf AEEY COXSIUERED. 

 THE ■J'UKKE I'KRIODS. 



1. The speculative period, in which, from a few rather imperfectly 

 determined facts of nature, general theories were evolved intended to 

 be applicable to all natural phenomena. It was a period in which 

 geology was not yet recognized as a distinct science and had hardly 

 reached the dignity of an adjunct to mineralogy. 



■2. The second period was that in which facts of observation had 

 accumulated sufficiently to establish geology on the basis of a distinct 

 science, but in which the method of reasoning from generals to par- 

 ticulars still prevailed. This was the first scientific period. 



3. The third period might be called the period of verification, in 

 which the theories already })ropounded were tested by experiment 

 or obser\ation. 



Such a classification is in the nature of things not susceptible of a 

 very definite demarcation either in point of time or in the assignment 

 to either period of individual o})inions or theories, l)ut the attempt to 

 make it, however imperfect and unsuccessful it may ])rove, will assist 

 us to form a clearer conception of the progress of liuman thought and 

 of the methods by which it has arri\ed at its present undi'rsfanding of 

 the ])articular branch of geological science which we are considering. 



THK SPECULATIVE PEKIOD. 



During the first or s])ecidati\'e jx-riod, which may b(» assumed to 

 have extended up to the close of the eighteenth century, or to the time 

 of Werner and Ilutton, the accumulation of accurately determined 

 facts that would bear on the theory of ore deposits was so extremely 

 limited that it may be assumed U) have exercised but little influence 

 on the development of the science l)eyond the suggestion it atl'orded 

 to later students of lines of investigation to be followed, and hence 

 may be passed over in a very cui'sory manner. 



In the speculations of this period which es[)ecially inlluenced the 

 development of opinion two general types may be distinguished: 



First. The broader theoi'ies of the cosmic philosophers with i-egai-d 

 to the formation of the earth, based more or less upon astronomic 

 data. 



Second. The special theoi'ies of mineral vein formation conceived 

 by individuals and based in tiic main on general conceptions which 



