In 1832 Schoolcraft, this time with Dr. Douglas Houghton as geologist to the party, again visited the Ontonagon River and the famous copper mass. On the way thither Dr. Houghton found copper embedded in the trap rocks on the western side of Keweenaw Point, and expressed the opinion that the "trap rock formanon was the original source of the masses of copper," and that-copper would probably not be found in that region in any other rock than trap, which Dr. Houghton at that time had seen only on Keweenaw Point.
During 1832 and 1833 Dr. Houghton visited the south shore of Lake Superior three times. Five years later he was appointed State geol ogist of Michigan and before his death, in 1845, had carefully examined the copper-bearing for mations, had ascertained their altitude and ex tent, had located a number of copper veins and had given to the world enough information to attract at once a large body of prospectors. The interest thus stimulated in the Michigan copper district soon led to the opening of some of its most famous mines. ;The success, of this new industry, as predicted by Captain Carver, was soon assured by the building of the canal at Sault Sainte Marie, which afforded the products of upper Michigan cheap transportation from the wilderness to market Geology.— The ((trap" rocks that constitute the greater part of the Lake Superior copper bearing formations are a series of old lava flows. All around the w.estern end of the lake through Michigan, Wisconsin and Minnesota, the trap beds extend for several miles in width, strilcing nearly parallel with the general trend of the shore line, and dipping always toward the lake, the dip being usually flatter near the shore than it is further back. Associated with these trap beds occur beds of reddish conglomerate or sandstone at different intervals, composed of the debris of the traps and associated more acid rocks.
On the.extrusion and flow of lava which has been under pressure, the pent-up gases in it expand near the surface of the flow like the bubbles in a glass of champagne, and as the surface of the flcrw rapidly congeals, each gas bubble forms a cavity or amy.gdule. In. the scoriaceous or amvgdaloidal portions of the lava beds, which are thus characteristic of the upper surface of the bed., though sometimes found at the bottom—also in cavities caused by chemical changes or by fractures, and occasionally in the conglomerates— are frequently found the deposits of native copper that characterize these formations. At different angles to the strike of
these beds fissure veins of later age are found, several of which, notably the Cliff, Phcenix and Central, proved also to be rich in copper, which in the Cliff mine carried with it a large amount of silver. The copper occurs either in minute flakes, or in more compact bodies from the size of a pin head or less to masses of 500 tons or more in weight, of shapes rounded like a boulder or branching irregularly through the parent rock-mass. Large masses seldom occur m the conglomerates, but in these beds individ ual boulders are often found entirely replaced by copper. In the early days of mining here it was the fissure veins that were sought and worked almost exclusively. They produced a large amount of mass in a small area of ground, and this could be made marketable without the expense of treatment in a stamp mill. By the time the fissure veins were begin ning to show signs of exhaustion the amygda loid deposits came more into notice and soon proved that while they were less rich than the veins, volume for volume, they were much more extensive, and, with the progress in mining methods, would probably prove profitable if worked on a large scale. The substitution of air drills for hand drilts and of high explosives for black powder helped to confirm this promise. Meanwhile the discovery of the Calumet con glomerate with its wealth of mineral had been made. As work on the fissure veins was grad ually being abandoned, the mines were in popu lar parlance grouped into two classes— amyg daloids and conglomerates. To-day, however, it is apparent that all so-called arnygdaloid cop per deposits are not properly so named and a further division is possible. Throughout the formations, especially along the upper surfaces of conglomerate beds, planes of wealcness have existed along which there has been more or less movement of the overlying formations. These planes of weakness not infrequently coincide with the amygdaloidal upper portions of the trap beds, both in strike and dip, and shearing movements along them have fractured the rock to so great an extent as to form a veritable breccia, in which copper was later deposited in worlcable quantities. Indeed the copper often occupies the interstices in these breccia beds, while the amygdules of the rock composing the breccia are entirely filled with minerals other than copper. In a case of this kind the term uamygdaloid* deposit, if not confusing, is at least inexact.