Dr. M. E. Waclsworth — Copper Deposits in Michigan. 21 



raindrops that fell, at ebb, upon the gently sloping beach. This 

 formation is known to you all through its affording the beautiful 

 Portage Entry or red sandstone (Devonian), so much used now in 

 buildiug. It is, however, with the central higher or plateau region 

 that we have the most to do with at present. 



You are all familiar with the descriptions or with the sight of the 

 lava beds of Vesuvius, or of Etna, or on Iceland, or on the Sandwich 

 Islands. You know how the lava flows onward towards the sea, 

 now rolling with a rough, ropy, clinkery surface ; and now gliding 

 with a comparatively smooth one. This Lake Superior plateau is 

 composed of a series of lava-flows like those of Kileaua, generally 

 smooth but sometimes clinkery. Let us imagine a large sheet of ice 

 extending over a lake — when from some cause a long fissure rends 

 it open on one side, and the water wells up through the sheet and 

 overflows the icy expanse. This overflow congeals ; the ice is again 

 rent in twain ; a new overflow takes place, and so on until the ice 

 continually sinking is piled up in successive thicknesses, hundreds 

 or thousands of feet. 



Let us now more exactly explain what has taken place in Northern 

 Michigan. The present promontory of Keweenaw Point once formed 

 the gently sloping tide- washed shores of a sea. Over this shore 

 poured the vast floods of lava, the same kind as now flows out from 

 Etna, Kileaua, and the majority of active volcanoes of the present 

 day. These flows, like those of Kileaua, were apparently quiet, and 

 not explosive like those of Vesuvius and Etna. At the time of the 

 outpouring of these vast floods of lava, the shores were gradually 

 sinking so that the congealed rock was exposed to the action of the 

 sea-waves. 



You all know of the effect of the storm-dashed waves upon 

 a rock-ribbed coast — how the rock is torn down and worn away, 

 and then piled up along the shores as a resulting mud, sand, gravel, 

 and shingle. In like manner our lava-flows, along the shores of 

 that great northern sea, where Lake Superior now is, were subject 

 to the alternate tide and storm-waves, and to the action of sun, 

 rain, wind, and frost. The result of all this must have been that 

 the exposed portions of these flows were buried under their own 

 debris, mingled with that of associated rocks. Besides the lava- 

 flows before mentioned we find other flows and masses, similar 

 in chemical composition to our granites, which, being much harder 

 and more enduring than the basaltic lavas, make up by far the 

 larger portion of the debris now visible. 



This i-egion, then, is composed of a series of interbedded basaltic 

 lava-flows, with their associated shingle, sand, and mud now forming 

 conglomerates, sandstones, and shales. 



In order to show more clearly what has happened since, let us 

 take a new metaphor, and look, upon all these layers as forming 

 a sort of marbled cake. Now let this cake be cut lengthwise 

 along one side of the thumb-like mass, the cut extending north-east 

 and south-west, nearer to the south-eastern side. Consider that 

 the north-western part has been lifted up at a varying angle from 



