Ill 



ON THE ORIGIN AND DEVELOPMENT OF ORE DEPOSITS IN 



THE WEST OF ENGLAND. 



By J. H. COLLINS, F.G.S. * 



Chap. III. — Rock Change as concekned in the Foemation 



OP Ore Deposits. 



Sec. 1. — General Gharacters of the Ore-Deposits. 



In presenting the following brief outline of the general 

 characters of the West of England ore-deposits, I must assume 

 that the main stratigraphical features of the district, and in 

 particular the relations of the granite and elvan to the stratified 

 rocks, are known to the reader. f 



Ore-deposits are frequently and conveniently classed as 

 contemporaneous (ore-beds, &c.), secondary or derivative (lodes, 

 i&c.), and detrital (placers, &c.) A more detailed, and at the same 

 time more accurate classification is that given below, which is 

 substantially the same as that adopted by Mr. J. A. Phillips 

 in 1884.+ 



1. Superficial, a. — Deposits formed by the mechanical 

 action of water, air, &c., in the denudation of mineral 

 regions. 



I. — Deposits of chemical origin formed at the 

 earth's surface by precipitation in lakes and back 

 waters, by organic agencies, or by the issue of mineral 

 springs or metalliferous vapours. 



2. Stratified, c. — Deposits of ore-substance constituting 

 the bulk of metalliferous beds, which have been formed 

 in situ by precipitation from aqueous solutions, and 

 subsequently but little altered. 



d. — Beds in which the ore-substance originally 

 deposited from solution has been subsequently altered, 



* Continued from the Journal, No. 36, p. 149. 



t For convenient summaries of this part of the subject see Mr. Henwood's 

 "Address" Journ. Roy. Inst. Corn., xiii., 1871, andihe present author's "Sketch 

 of the Geology of Central and West Cornwall," Proc. Geol. Assoc, x, 3, 1887. 



I " Ore-deposits," p. 3. 



