Wadsworth.] 200 [May 7, 



eruptive and altered rocks. From what has been said the general 

 alteration of rock masses, and the partial localization of their 

 contained mineral matter by percolating waters, would appear 

 to give rise to a large proportion of the ore deposits found 

 in veins, segregations, and impregnations. 



Instead of the mineral matter taken up by the percolating 

 waters being deposited in the rock again or in contiguous cav- 

 ities, it may be borne far away, appearing in spring, river, lake, 

 and ocean waters, and in deposits laid down by them, precipitation 

 taking place wherever the proper conditions exist. 



If we start, as all geologists do, with the belief in an originally 

 hot fluid globe, all rocks must have been derived, primarily, from 

 fluid material. The detrital rocks would naturally partake of the 

 characters and changes of the rocks from which their material 

 came ; while in the chemically and organically formed rocks there 

 can be readily suggested, in accordance with their special condi- 

 tions of formation, agencies for the precipitation of useful ores 

 throughout their mass, — the precipitated ores being gathered up 

 subsequently by the percolating waters. 



In order to draw any conclusions concerning the reliability of 

 deductions regarding the source of the ore, based on the analy- 

 sis of the minerals in the country rock, adjacent to an ore deposit, 

 it is necessary to look into the question of the origin of these 

 minerals. It has been found, that if the point of consolidation of 

 rocks be taken as the reference point, their minerals naturally fall 

 into three classes : 1. Those of prior origin — foreign. 2. Those 

 produced by solidification (crystallization) — indigenous. 3. 

 Those produced later by alterations in the rock mass, or b} r infil- 

 tration — alteration or secondary. 1 The first class can be conveni- 

 ently separated into two divisions : 



1. The minerals that are characteristic of the rock whatever 

 may be its locality or age. 



2. Those that are accidental, as for instance, fragments caught 

 up during the passage through or over another rock. 



Any rock may have in it all three classes or only one or two as 

 the case may be. A few minerals may be cited in illustration : 

 olivine, in the peridotites is an indigenous mineral, but in the ba- 

 salts is foreign, although generally characteristic of them. Ser- 

 pentine, when not an infiltration or veinstone product, is always a 



i Bull. Mus. Comp. Zool., 1879, V, 277, 278. 



