REPORT OF THE DIRECTOR igi2 2^ 



character, in fact, all gradations from the one rock to the other may 

 be found. 



The limestones and schists have a northeasterly strike and are 

 upturned at a high angle. The limestones of this section carry 

 abundant impurities, though elsewhere the same series may be 

 nearly free from admixture. The principal foreign minerals are 

 silicates, most commonly tremolite, serpentine and talc. They are 

 either scattered in small aggregates, or they form nodules, bands 

 and veinlike bodies of practically solid silicates. The limestones 

 are magnesian and in the vicinity of the ore bodies show the effects 

 of solution and decomposition by ground waters. The circulation 

 of water has been facilitated apparently by the broken, shattered 

 nature of the rock which has undergone severe compression and 

 more or less differential movement. The process of dolomitization 

 and silication has preceded for the most part the introduction of 

 the ores, but it may have resulted from the same agency, that is 

 by the transporting of silica and magnesia held in solution in 

 meteoric or deep-seated waters. 



The zinc blende occurs in lenses and bands and also as scattered 

 particles within the limestone. The deposits have the appearance 

 of replacement bodies rather than the fillings of open fissures or 

 cavities. In most places, the borders of the richer bands are not 

 sharply defined, but are in the nature of transition zones which 

 shade off gradually into the limestone. The internal structures are 

 not those characteristic of open-fissure fillings as there is no appear- 

 ance of crustification or of drusy cavities lined with crystallized 

 minerals- The compact granular nature of the ore furthermore 

 suggests deposition at considerable depth and under pressure. 



The recent development work at Edwards has disclosed some 

 interesting features in regard to the deposition of the ores which 

 are the subject of current study. The problem as to the derivation 

 of the ores seems to be interrelated with the partial silication of 

 the limestones which has led to the formation, in the first place, 

 of tremolite. This mineral has changed over to talc, more or less 

 completely, through normal weathering or, which appears more 

 likely, as the result of decomposition brought about by the later 

 stages of the underground circulations that deposited the ores. 

 The serpentine in larger part, however, seems to have formed 

 directly, that is deposited as such from solution and not originating 

 as an alteration product of an anhydrous mineral. Some of the 

 serpentine is certainly later than the metallic minerals, as shown by 

 the veins and stripes of the colloidal variety which intersect the ore. 



