POTSDAM PERIOD. 195 



and in general little altered, and the tilting which is observed ap- 

 pears to have taken place in a later period. These remarks apply- 

 to the larger part of the Potsdam rocks about Lake Superior. But 

 on Keweenaw Point, the famous copper-region of Lake Superior, 

 the sandstones of this period are associated with trap, — an igneous 

 rock that was ejected through fissures opened in the earth's crust ; 

 and these trap ejections have added vastly to the accumulation, so 

 that in some places about Keweenaw Point the alternations of 

 sand-stone, conglomerate, and trap rocks make a thickness of eight 

 to ten thousand feet. Some of the conglomerate (according to 

 Foster & Whitney, and Owen) seems to be made of volcanic 

 scoria, like the tufa of modern volcanoes, as if the ejections were 

 submarine and the cool waters had shattered the hot rock to frag- 

 ments and so made the material of the conglomerate ; and, as many 

 of the masses are not rounded, these authors infer that it was piled 

 up rapidly during the igneous action. Dr. D. D. Owen represents the 

 trap as often in layers alternating with shale and other rocks, indi- 

 cating eruptions at different times. The trap rocks of Lake Supe- 

 rior present many scenes of basaltic columns of remarkable gran- 

 deur. Some of them are represented and described in the Geolo- 

 gical Report on Wisconsin, Iowa, and Minnesota, by Dr. Owen. 



The native copper of the Lake Superior region is intimately 

 connected in origin with the history of the trap and sandstone. 

 The copper occurs in irregular veins in both of these rocks near 

 their junction ; and whenever the trap was thrown out as a melted 

 rock, the copper probably came up, having apparently been derived 

 from copper-ores in some inferior Azoic rocks through which the 

 liquid trap passed on its way upward. The extent to which the 

 rock and its cavities are penetrated and filled with copper shows 

 that the metal must have been introduced by some process before 

 the rock had cooled. The nature of this process, and the condition 

 of the metal (whether as a salt or other compound in solution, or 

 in vapor), have not yet been ascertained. One great sheet of copper 

 which has been opened to view in the course of the mining was 

 forty feet long, and weighed, by estimate, two hundred tons. The 

 copper is mixed with native silver; and some specimens are 

 spotted white with the more precious metal. 



In addition to copper, the rocks contain the usual trap minerals, 

 — zeolites, datholite, calcite, quartz; and some calcite, datholite, 

 and analcime crystals were formed about threads of copper. 



Besides the disturbance connected with the Lake Superior 

 rocks, there were great oscillations of level over the continental 

 seas, causing the changes in the depositions from sandstone to 



