COLLECTIONS IN ECONOMIC GEOLOGY AND METALLURGY. 93 



THE DELAWAKK MINK. 



The Delaware Mine, situated iu the upper part of the Keweenaw point, 

 Michi<;au, 35 miles from Portage Lake, was selected to illustrate the 

 conglomerate veins. The first workings on this location were directed to 

 the extraction of copper from the transverse veins carrying mostly mass 

 copper, but were unsuccessful. In 1881 attention was directed toward the 

 conglomerate bed, and a new plant commenced for actively working it. 

 The mine has been equipped in the best manner, and is well provided 

 with all the machinery for extensive working. It has been opened to 

 a depth of 1,600 feet, and for a length of 2,150 feet ; it is provided with 

 three shafts for hoisting the ore, and the general development is far in 

 advance of the actual working. 



A section of the strata at this point shows first, greenstone 500 to 

 GOO feet thick, dipping 24 degrees to the north ; then 7 feet of a shattered 

 black trap ; between this and the conglomerate there is a selvage seam 

 of clay 2 inches thick. The conglomerate bed is 25 to 28 feet thick, and 

 below is an amygdaloid bed 50 to 60 feet thick. Beyond this there is a 

 succession of the ordinary country rocks. 



The conglomerate is generally coarse, although it varies from a sand 

 to boulders of a cubic foot or more. The pebbles are mostly red and 

 brown quartz porphyry s. Occasionally, however, there are pebbles of 

 amygdaloid, and near the base of the bed where the conglomerate is 

 more loosely cemented, the number increases. These amygdaloid peb- 

 bles appear to have come from the bed below the conglomerate. There 

 is a marked stratification of the conglomerate, from the variation in the 

 size of the pebbles. The sandstone strata are generally thin, and are 

 quite local. They appear to contain a large proportion of feldspathic 

 material. 



The copper-bearing layer of the bed is generally found within 8 or 

 10 feet of the base, although the first 2 feet above the foot wall is gen- 

 erally barren. Above this 2 feet the copper is distributed through a 

 single layer 5 or 6 feet thick or through several thinner layers separated 

 by barren material. Above these layers little copper is found, although 

 small masses have been frequently found along the contact between the 

 top of the conglomerate and the greenstone. The general character 

 of the copper-bearing layers is the same as the barren layers, except 

 that they are somewhat more open in structure. The cop[)er lias been 

 deposited in the cementing material between the grains, and some of 

 the richest material is sandstone cemented by copper. Coj)per has not 

 been found in the interior of the quartz phorphyr}^ pebbles, but has 

 been found in the amygdaloid i)ebbles. 



Near the bottom of the bed interesting masses of copper are found 

 which have evidently been deposited in cavities left by the dissolving 

 out of amygdaloid pebbles. In some cases the copj)er forms a perfect 

 cast of the original pebble, while iu others the deiiositiou of copper has 



