4IO PETROLOGICAL ABSTRACTS AND REVIEWS 



shale are sills and dikes of quartz diabase, to the northwest of which lie the 

 interbedded basaltic flows. The quartz-diabase is a fine-grained dark greenish 

 rock becoming very fine-grained at the contact ; it may bear phenocry sts of 

 augite, attaining a length of a centimeter; the texture is diabasic or ophitic, 

 the labradorite being quite zoisitized. 



The basalt is brecciated or tuffaceous and amygdaloidal, hence the flows 

 were probably subaqueous; under the microscope the basalt breccia shows as 

 a perlitic, dark greenish glass. 



Guild, F. N. "A Microscopic Study of the Silver Ores and Their 

 Associated Minerals," Econ. GeoL, XII (1917), 297-353, P^s- n- 



This paper represents a summary of rather extensive work done by Mr. 

 Guild on silver ores from many localities, both here and abroad. The charac- 

 teristic early minerals of silver deposits are pyrite, sphalerite, and arsenopyrite. 

 These are chiefly hj^pogene in origin, though the first may also occur as a 

 secondary mineral. They are commonly followed by the deposition of argen- 

 tiferous galena and tetrahedrite; the order of deposition is arsenopyrite, pyrite, 

 sphalerite, tetrahedrite, chalcopyrite, and galena. The later silver minerals 

 that are discussed include stromeyerite, which exhibits exceedingly close 

 "pseudo-eutectic" intergrowths, that may actually be eutectic in nature, 

 pyrargyrite and proustite, stephanite, polybasite, argentite, and native silver. 

 The latter certainly forms in some cases as a result of the breaking down of 

 complex silver minerals. Other sources of silver are also briefly considered — 

 cerargyrite and the rarer minerals huntlinite (silver arsenide), dyscrasite (silver 

 antimonide), brogniardite (a complex sulphide of sflver, lead, and antimony), 

 and schirmerite (bismuth-silver-lead sulphide). Variously associated with the 

 silver are chalcopyrite, bornite, chalcocite, and covellite. Commonly associ- 

 ated cobalt and nickel minerals are smaltite, niccolite, and breithauptite. 



The predominant gangue minerals found with silver are carbonates — notably 

 in the lead-silver type of ores; this indicates a tendency for such ore-minerals 

 to come from neutral or alkahne solution, as carbonates could not be developed 

 in acid solutions. The gangue minerals mentioned are calcite, dolomite, 

 siderite, rhodochrosite and barite. 



Excellent hints as to methods for distinguishing silver minerals micro- 

 chemically are presented, a subject so far neglected only too often. Good 

 manuals for the ore-geologist are indeed rare, and good manuals deaUng with 

 the microscopy of silver minerals are wholly lacking. 



The paper contains interesting discussions of graphic intergiowth, of 

 "enrichment" of silver ores, and of the composition of some important silver 

 minerals. A table presents the investigator 's ideas regarding the sequence of 

 deposition of double salts of silver and other metals. 



