xn. 



THE ORIGIN OF METALLIFEROUS 

 DEPOSITS. 



This paper, unlike the others in this collection (with the exception of IV.), was a 

 lecture to a general audience, given before the American Institute of New York, in 

 May, 1872, and reported for their Proceedings. It is reprinted here because it states, 

 though in a familiar manner, certain views which the author believes to be important. 

 The following extract from a review of American Geology in the American Journal 

 of Science for May, 1861 (a part of which is published as Essay V. of this volume), is 

 prefixed as a concise statement of some of the points in the lecture. 



" THE metals .... seem to have been originally brought 

 to the surface in watery solutions, from which we conceive 

 them to have been separated by the reducing agency of organic 

 matters in the form of sulphurets or in the native state, and 

 mingled with the contemporaneous sediments, where they occur 

 in beds, in disseminated grains forming fahlbands, or are the 

 cementing material of conglomerates. During the subsequent 

 metamorphism of the strata these metallic matters, being taken 

 into solution by alkaline carbonates or sulphurets, have been 

 redeposited in fissures in the metalliferous strata, forming veins, 

 or, ascending to higher beds, have given rise to metalliferous 

 veins in strata not themselves metalliferous. Such we conceive 

 to be, in a few words, the theory of metallic deposits ; they 

 belong to a period when the primal sediments were yet impreg- 

 nated with metallic compounds which were soluble in the per- 

 meating waters. The metals of the sedimentary rocks are now, 

 however, for the greater part in the form of insoluble sulphurets, 

 so that we have only traces of them in a few mineral springs, 

 which serve to show the agencies once at work in the sedi- 

 ments and waters of the earth's crust. The present occurrence 

 of these metals in waters which are alkaline from the presence 



