28 SILURIAN SYSTEM. 



posed largely of conglomerate layers and large-sized pebbles, indicating strong cur- 

 rents of water. The flows visible upon the borders of the lakes were forced through 

 fissures by volcanic energies. The copper which occurs in the conglomerates, amyg- 

 daloids, epidote veins, and otherwise, is supposed to have been precipitated from 

 water holding it in solution, or leached from detrital rocks where it was originally 

 deposited in a sulphureted form. R. D. Irving, who has studied closely the cop- 

 per-bearing rocks of this region, says the explorer for transverse veins should bear 

 in mind that epidote, prehnite, and chlorite are favorite associates of copper, while 

 laumonitic veins, and those bearing a predominating quantity of calcite, are not so 

 rich ; that a wide vein in amygdaloidal or other soft rock will pinch to a mere 

 seam within the massive and compact layers; and in sandstone and conglomerate 

 deposits the valuable belts have been found where the conglomerate is overlaid 

 with trap, or in sandstone very rich in basic detritus. Any of the conglomerate 

 seams from Keweenaw Point to Minnesota may be cupriferous. All of the upper 

 division of the series is noncupriferous, except the Nonesuch sandstone belt in the 

 Porcupine Mountains ; and all the belts and areas of acid rocks, such as the central 

 area of the Porcupine Mountains, and the great spread of red rock in the Brule" 

 Lake country in Minnesota, and all belts and areas of coarse-grained basic rocks, 

 such as the great area of coarse gabbro in the Bad River region in Wisconsin, and 

 the similar area which occupies the belt of country from Duluth to Brule Lake, 

 are also noncupriferous. The slates and quartzites of the Taconic System which 

 lie below the Cupriferous series on the north shore of Lake Superior, have been 

 called the Animikie Group. About three-fourths of the great thickness of the 

 rocks is referred to volcanic overflows, and does not, therefore, belong to the geolog- 

 ical column, the whole of which is the result of sedimentary deposition. 



CHAPTER IV. 



SILURIAN SYSTEM. 



§ 52. In 1833, Sir R. I. Murchison, in a memoir read before the Geological 

 Society of London, divided the fossiliferous rocks below the Devonian into six 

 Groups. He founded this subdivision upon the fossils, and mentioned such species 

 as were then defined. This was followed in the succeeding year by other memoirs, 

 and in 1835 he concluded all these Groups might be placed in one System, and in 

 honor of the ancient tribe of Silures, who inhabited Wales, he named it the Silu- 

 rian. He placed three Groups in the Upper Silurian, and three in the Lower 

 Silurian. Before this time no knowledge of the order of the strata had been ascer- 

 tained, and hence he is entitled to the credit of the name. Subsequently palaeontol- 

 ogists found his discoveries were world-wide in their application, and it was not 

 long until the distinction between the Upper and the Lower Silurian had been 

 observed in North America, and the base of the Lower Silurian had been recognized 

 in the Potsdam sandstone, and the base of the Upper Silurian in the Medina 

 sandstone. 



§ 53. Some years later, the word Cambrian was applied in England to the rocks 

 which belong to the Lower Silurian, and to inferior strata which are the equiva- 



