1914] Lawson: Ore Deposition by Meteoric Waters 



227 



move laterally goes without saying. It may be also again noted that 

 the waters thus coming in contact with the hot igneous mass would 

 not be wholly dependent upon it for the supply of mineral matter. 

 The sedimentary rocks would also yield their quota. 



Deposition of Ores in Inclusions and Salients. — Lindgren's next 

 objection is that "ores are generally found at limestone points pro- 

 jecting into the intrusive mass or in slabs of limestone swimming in 

 the igneous rock. It is difficult to see why the circulating atmospheric 

 waters should seek such places by preference. In many deposits there 

 is no evidence of fractures or paths which could have been followed 

 by the water." Here my critic leaves the a priori method of attack 

 and descends to the safe field of observation, where of course he is 

 very much at home. It is to be regretted, however, that he did not 

 cite some specific instances of ores formed in inclusions in igneous 

 rock. That ores may occur in slabs of limestone swimming in igneous 

 rock I do not deny, but it is incumbent upon Professor Lindgren to 

 show that the ore was not formed when the slabs were an integral 

 portion of the invaded country and before they were subsequently 

 torn off and engulfed as inclusions as the irrnptive process advanced. 

 The same inconelusiveness inheres in the statement as to ore being 

 generally found at limestone points projecting into the intrusive mass. 

 An intrusive mass as it invades a limestone country by stoping or 

 resorption encroaches upon the zone which may have previously been 

 the seat of ore deposition and so may bring mineralized points into 

 the transgressive contact. This explanation involves, of course, the 

 possibility of the garnet zone and some of the ore associated with it 

 having been formed by circulating heated waters during the intrusion 

 if the latter be a single act. But until such occurrences are particu- 

 larized I shall be inclined to doubt their generality and to regard 

 them as exceptional. The general condition appears to be that the 

 ores were formed after the consolidation of the intrusive mass. Thus 

 Barred, 17 speaking of the ores at Marysville, says: 



"Some little time elapsed between the solidification of the batholith and the 

 opening of the vein fissures, since not only had there been a period of aplitic 

 and pegmatitic injections, but a number of later intrusions of porphyry had 

 occurred previous to the formation of the vein fissures. It would appear prob- 

 able, therefore, that the fissure veins, if indeed they are contraction effects, 

 belong not to an initial stage but to the final stages of cooling. ' ' 



it U. S. G. S. Professional Paper No. 57, p. 106. 



