128 



BULLETIN OF THE 



scnce of any characters showing intrusion of the traps between the dif- 

 ferent beds. The extent of these old lava flows, excej^t as seen along 

 their upturned edges, is not known, for they have been followed by 

 mining along their hicline some 2,100 feet only. Judging from their 

 length, their width must be considerable. In thickness they partake 

 of the same irregularities which any similar flow has. Locally the thin- 

 ner and more scoriaceous flows are known as " amygdaloids,^' the thicker 

 and more compact ones as *' traps," and the thickest one on the western 

 side as "greenstone." The difierence between these several forms of 

 old basalt seems to be occasioned only by the variable amount of lava 

 erupted at different times, and the slightly dilTerent conditions to which 



the flows were subjected. 



Most of these old basalts are directly covered by succeeding flows, 

 followung after greater or lesser intervals of time ; but part, as remarked 

 above, are covered by conglomerates and sandstones. These conglom- 

 erates and sandstones show, by the rounded and water-worn character 

 of their constituent pebbles and grains, that they are beach deposits. 

 The surface of the underlying basalt is smoothed as by water action. 

 The overlying conglomerate is made up at its base of })asaltic mud and 

 pebbles, derived from the underlying rock, and mixed with the felsitic 

 mud and pebbles of which the conglomerates are chiefly composed. The 

 trappean mud and pebbles diminish, or are entirely wanting, as wc re- 

 cede from the underlying trap. That the basalt is a metamorphosed 

 sandstone, as is often contended by mining men, is disproved by the facts 

 given above, and by the further facts, that there is no gradual, but an 

 abrupt, passage between the two ; that all fi'agments of sandstone caught 

 up by the trap are baked and indurated by the heat, but show no signs 

 of passage; and, lastly, that it would demand the conversion of an acidic 



rock into a basic one. 



From the conditions given above it would not be surprising to find the 

 lava flows locally limited at any point, and partially or even entirely de- 

 nuded, and replaced, in part or as a whole, by conglomerates and sand- 

 stones. This would depend, of course, upon the position of the shore 

 line, and upon the conditions to which the basalt was subjected during 

 the time of its flow, or since. A still greater variability is to be looked 

 for in the conglomerates and sandstones. All these conditions are to be 

 taken into consideration in the mining of these old lava flows and their 



associated conglomerates. 



In the Portage Lake and Keweenaw Point districts there are mined at 



present four forms of deposits: 



