182 BULLETIN 109, UNITED STATES NATIONAL MUSEUM. 



course they necessarily cut across the dikes of trap which lie between 

 the strata. These veins all belong to one epoch. They present com- 

 plete uniformity of characters — when they traverse continuous ranges 

 of the trap, they are regular in course and direction: but in a single 

 knob they are irregular and undefined. They contain metalliferous 

 materials which " will hereafter become of very considerable prac- 

 tical importance." It is true that native copper occurs " occupying 

 the joints or natural septa of the greenstone, but in these instances 

 the amount of metal is comparatively small, and, with one or two 

 exceptions, I have invariably been able to establish some connection 

 between the native metal occupying these joints and the termination 

 of some metalliferous vein that traverses other portions of the rock 

 not far distant." 



Arguing that the richest veins would be found along the line of 

 contact of the trap with the sedimentary rocks, great pains had been 

 taken to trace that line across the country. It was found to diverge 

 gradually from the lake in its westward extension. The theory was 

 confirmed that the trap is the source of the copper occupying the 

 veins, since as soon as the veins enter the sandrock they contain no 

 ores but those of zinc and iron (p. 54). 



The metalliferous veins, Doctor Houghton continues, seem to orig- 

 inate in the northern or amygdaloidal trap. They cross the trap 

 range nearly at right angles to its axis, and frequently continue the 

 same course across the sedimentary rocks. But the width of the vein 

 is greatly diminished in the greenstone. 



It increases in widtli rai>idly as it passes across tlie amygdaloid, and at or 

 near the junction between the amygdaloid and the sedimentary rocks, it will 

 frequently be found to have attained a thicliness of several feet, while in its 

 passage across the sedimentary rocks, it is usually either further increased in 

 width or becomes so blended v^^ith rock itself as to render it difficult to deflnp 

 its boundaries. 



In the amygdaloid the veinstone is mostly quartz; in the con- 

 glomerate and sandstone, it is mostly calcareous, and finally ceases to 

 be metalliferous. The metalliferous character is most fully de- 

 veloped near the line of junction of the trap and sedimentary rocks. 

 The most abundant metallic minerals are the several ores of copper. 

 Very frequently native copper occurs, and very rarely, silver. The 

 native copper is often disseminated through the quartzose veinstone, 

 the rocky matter embraced in the veinstone and the amygdaloid and 

 conglomerate, sometimes to the distance of 2 or 3 feet, completely 

 filling the cells of the amygdaloid. In other portions of the vein 

 the native copper is concentrated in larger masses, constituting 

 bunches and strings; and in some places the walls of the veins are 

 wholly made up of native copper. In these places the ores of copper 



