THEORIES OF ORE DISPOSITION IlISTOKK'ALLY CONSIDERED. 311 



ev(Mi iitloinpt a (lescription of the niodo of occiirreiK-e of Uio oi-es, 

 nuu'li less spcciilale on their origin. 



The historic lime liere coiiteiiiphited may l>e divided, in a general 

 way, into three periods, aeeordinii- to the prevailinir method by which 

 the views then current were arrived at. 



DEVELOr.MENT OF KXOWLKlXii: 11 ISTOKK 'Arj,Y CONSIDERED. 



THE TUKKK I'KKIODS. 



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

 vleternnned facts of nature, general theories were evolved, intended to 

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

 geology was not 3^et 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 

 accuunilated 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 propounded were tested by experiment 

 or observation. 



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 opinions or theories, but the attempt to 

 make it, however imperfect and unsuccessful it va-Aj prove, will assist 

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

 of the methods by which it has arrived at its present understanding of 

 the particular branch of geological science which we are considering. 



THE SPECULATIVE PERIOO. 



During the first or speculati\'e jieriod, which may be 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 tluH)ry of ore deposits was so extremely 

 limited that it may be assumed to have exercised but little influence 

 on the (leveloi)ment of the science beyond the suggestion it afforded 

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

 may be passed over in a very cursory manner. 



In the speculations of this period wdiich especially influenced the 

 development of opinion two general types may be distinguished : 



First. The broader theories of the cosmic philosophers with regard 

 1o the formation of the earth, based more or less upon astronomic 

 data. 



Second. The special theories of mineral vein formation conceived 

 by individuals and based in tlie main on general conceptions which 



