390 ME. E. EUTLEY ON THE OEIGIN OF [Aug. 1 894, 



of limestone, the latter having since been in great part dissolved and 

 replaced by quartz, which has taken up some of the gold it contained. 

 The quartz envelops a residue of the limestone, tbe marginal portions 

 of the included limestone-patches showing that the process of dis- 

 integration was going on when it was arrested by the solidification 

 of the silica (fig. 2, p. 389). 



The brisk evolution of bubbles when a fragment of the rock is 

 placed in dilute hydrochloric acid indicates that the carbonate is, 

 in this case, not dolomite, but calcite. A drop of the solution, when 

 treated with chloride of ammonium and ammonia, and phosphate of 

 soda added, yields a few microscopic crystals of the ammonio-phos- 

 phate of magnesia ; but this magnesia is doubtless derived from a 

 pale greenish substance, which is present here and there in the 

 calcite, and which may be regarded as probably some variety of 

 chlorite. 



We have, then, in this rock a case in which a quartzite of com- 

 paratively coarse microscopic texture has originated as a siliceous 

 replacement of a limestone, just as I believe the fine-grained 

 Ouachita stone and the still finer Arkansas stone are the siliceous 

 replacements of dolomites or of dolomitic limestones. 



Although the association of gold with calcite is unusual, it is 

 by no means unknown. Thus, the late John Arthur Phillips, in 

 describing the St. David's Lode (Clogau Mine, North Wales), says :— 

 " It is chiefly composed of quartz and calcite, the latter mineral 

 sometimes forming masses of several feet in width ; where the 

 calcite assumes the appearance of a friable and granular marble, it 

 not unfrequently contains gold, but when, on the contrary, it 

 becomes foliated or is coarsely granular, that metal appears to be 

 entirely wanting." 1 



It may be that the evidence adduced in support of the views 

 advanced in this paper will not prove conclusive to all minds. Those 

 views were, in fact, laid before this Society in a former paper, 2 to 

 which this may be regarded as a sequel. It is but a very imperfect 

 sequel, since I hoped to have been able to discuss the differences 

 between concretionary and residual limestone-nodules, but, in the 

 absence of suitable material to work upon, this portion of the subject 

 could not be dealt with. 



In conclusion, I am anxious to express my profound appreciation 

 of Mr. Griswold's work, although I have occasionally taken the 

 liberty to criticize it. To do it justice would, indeed, be no easy 

 task. His book is a mine of facts, a storehouse of well-arranged 

 and valuable information. Upon several of the points on which I 

 have ventured to differ from Mr. Griswold, he has himself expressed 

 doubt, and I have therefore felt the less diffidence in attempting to 

 explain that which seemed obscure. 



1 ' Ore Deposits,' p. 203. 



2 ' On the Dwindling and Disappearance of Limestones,' Quart. Journ. Greol. 

 Soc. vol. xlix. (1893; p. 372. 



