COPPER. 139 



2.04.09. Example lib. Llano County, Texas. Impregnations in 

 granite, and veins with quartz gangue in granite, carrying car- 

 bonates above, but sulphurets and tetrahedrite with some gold and 

 silver below. Contact deposits between slates and granite are also 

 known. It is not demonstrated as yet whether the ores are to be 

 actually productive. l 



2.04.10. Example 18. Keweenaw Point, Mich. Native cop- 

 per, with some silver, in both sedimentary and interstratified 

 igneous rocks of the Keweenawan system. The metal occurs as a 

 cement, binding together and replacing the pebbles of a porphyry 

 conglomerate ; or filling the amygdules in the upper portions of 

 the interbedded sheets of massive rocks ; or as irregular masses, 

 sometimes of enormous size, in veins, with a gangue of calcite, epi- 

 dote, and various zeolites; or in irregular masses along the contacts 

 between the sedimentary and igneous rocks. 



2.04.11. The rocks of the Keweenawan system are most 

 strongly developed on the south shore of Lake Superior, especially 

 in Keweenaw Point, which juts out northwesterly, cutting the lake 

 into t\vo nearly equal portions. They extend some distance east 

 and west and are also known on the north shore. They consist of 

 sandstone and thin beds of conglomerate, interstratified with sheets 

 of diabase, both compact and amygdaloidal, and of melaphyre. 

 They are succeeded on the east by the Eastern Sandstone, which 

 on the south shore in some places abuts unconformably against 

 them, and in others passes under them from an overthrust fault. 



On Keweenaw Point they dip northeasterly and pass under 

 Lake Superior to reappear with a southeasterly dip on Isle Royale 

 and the Canadian shore. Western Lake Superior occupies this 

 synclinal trough. In Keweenaw Point the dip is greatest on the 

 southeast, being about 60 at Hancock. To the northwest it 

 gradually flattens to 30 or less on the lake shore. (For the gen- 

 eral geology of the neighboring region see under Example 9.) 



It is interesting to note that the early investigators of the 

 geology of this country drew a parallel between the sandstones 

 and traps of Lake Superior and the similar Triassic deposits of the 



Col. State School of Mines, 1887, p. cii. A. W. Rogrers, '-The Mines and 

 Mills of Gilpin County, Colorado," M. E., II. 23. Further references will 

 be found under "Silver and Gold in Colorado." 



1 T. B. Comstock, First Ann. Rep. Texas Geol. Survey, 1889, p. 334. 

 W. H. Streeruwitz, in Mineral Resources of the U. S., 1884, p. 342. 



