SALMON — OX THE FORMATION OF OEE-TEINS. 393 



have originated pm^ely by infiltration, altlioiigli tlie fii^sf lias been 

 found in the Freiberg copper- veins and in the Annaberg silver ones ; 

 and the spaces, too, of felspar-crystals in the granite of Corn-wall can 

 evidently have only been filled by infiltration, or snblimation. These 

 metals belong, perhaps, principally to certain massive rocks as 

 accessory constitnents, out of the detritus of which are often derived the 

 portions of those metals found in washings. Originally they occuiTed 

 in common with the other metals, but it seems to be a peculiar 

 characteristic of their natm^e that they do not occur in purely 

 infiltration- veins. 



K'OTES ON THE ABOVE MeMOIR. 



As Professor Cotta pre-supposes on the part of his readers a 

 general acquaintance Tvith the subject of nuneral-veins and the diffi- 

 culties which beset the solution of their theory, the force of many of 

 his observations may not be remarked by those to whom this know- 

 ledge is not famihar. For the benefit of such I have jotted do^^^l 

 the follovking notes which, while serving to elucidiate this memoir, 

 may also be useful as indicating some of the difiiculties that beset the 

 subject, and some of the more important preliminary problems 

 necessary to be solved. 



I. Uelcttion between crystalline and eruj^tive roclcs and metalliferous- 

 deposits. — Prof. Cotta's reasoning is entirely founded upon the gene- 

 rally observed relation between ciystalline and eruptive rocks and 

 metalliferous deposits. That this relation is a nearly constant fact 

 seems to be beyond doubt, although there are occasional exceptions — 

 not, however, sufficiently numerous to destroy the force of the ob- 

 served relations, which have passed into a proverb in most mining- 

 districts. But merely obsei'ving the fact of this relation is one thing, 

 and accounting for it in a satisfactory manner is another. The first has 

 been done universally ; the second has never been seriously attempted, 

 and probably, in the present state of our knowledge, is a problem im- 

 possible of solution. The most Ave can do is, as Prof. Cotta has done, 

 to suggest such a probable hypothesis, as may be useful in giving a 

 definite direction to oui' ideas and to our investigations. 



II. Wlience are tJie metaJ-contents of ore-de^josits derived ? — This 



