ELY GEEENSTONE. 139 



all of the cases observed, almost purely superficial, extending at most only a 

 few feet down into the rocks. Such occurrences hav.e led to considerable 

 waste of money in the sinking of prospect holes. The joints and gashes in 

 numerous places throughout the district have been found filled more or less 

 completely with quartz, occurring both as vein quartz and in a saccharoidal 

 condition, more or less intimately mixed with carbonates. In several places 

 large veins of quartz traverse the greenstones, but minute ones are more 

 common. The largest veins have been prospected for gold, and several 

 gold mines, so called, have been opened along them. Where the quartz 

 veins are mixed with carbonates the carbonate usually carries a consider- 

 able content of iron, so that on weathered surfaces such vein deposits are 

 quite ferruginous. This infiltrated carbonate-bearing material" is especially 

 common in the interstices and in the schistose matrix between the ellipsoids. 

 In this same position, and apparently but a further alteration product 

 of such secondary carbonate-bearing deposits, is a white, black, or purplish 

 chert, and less frequently a red jasper. Not uncommonly, also, the non- 

 ellipsoidal greenstone near the jasper belts contains irregular bunches and 

 lenticular areas, varying in size, of rather coarse white and black chert, 

 with more rarely the true red jasper. Such deposits are certainly, in many 

 cases, composed of infiltration products brought from overlying formations 

 and deposited in the interstices of the basal greenstones. They are never 

 of very large size, and it is of course useless to prospect in such places for 

 paying ore bodies. 



Certain of the macroscopic structures, namely, the amygdaloidal, the 

 spherulitic, and the ellipsoidal, mentioned above as being present in these 

 greenstones, are very well developed, and on account of this, but chiefly as 

 they offer a clew to the mode of formation of the greenstones, they are of 

 some interest and Avill be described m detail. 



THE AMYGDALOIDAL STRUCTURE. 



Of the three structures mentioned above the most common is the 

 amygdaloidal. This is rarely seen in the very coarse greenstones, but is 

 usually present in the finer-grained varieties. This structure is most notice- 

 able on weathered sm-faces, whel'e it may be recognized by the presence of 

 rounded or oval spots scattered over the rock. On examining the internal 



fflMon. U. S. Geol. Survey Vol. XXXVI, 1899, pp. 130-135. 



