10 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
the associated sandstones, and in part as protrusions ‘in vast irregular 
masses, forming conical or dome-shaped mountains,” or ‘‘ continuous lines 
of elevation.”' While they class the amygdaloids and traps together, they 
do not appear to have distinctly recognized that the former are the vesicular 
upper portions of the lava flows. 
Houghton regarded all the traps as intrusive, and the amygdaloids 
as semi-fused sedimentary matter or interfused eruptive and sedimentary 
matter,” in which view he was followed by Jackson.’ Similar views as to 
the intrusive origin of the traps were held by a few others at an early date, 
but the only one of these holding these views who had made any extensive 
field explorations was Norwood,‘ who in his account of the Minnesota coast 
represents a large proportion of the bedded crystalline rocks of that coast 
as intrusive; but even he does not regard all of the traps as having had this 
origin. The amygdaloids he appears to look on, for the most part, as sedi- 
ments altered by igneous action, his general term for them being “meta- 
morphic shales.” A few of these rocks he seems to regard as volcanic 
tufas, an origin which was somewhat doubtfully assigned by Hunt to all 
the amygdaloids.2 N. H. Winchell has recently revived the peculiar views 
of Norwood as to the origin of the amygdaloids of the Minnesota coast.® 
Of others who have written on Lake Superior geology the larger 
number have maintained the origin of the traps as lava flows. Among 
them may be mentioned Logan,’ Hunt,*® Macfarlane,’ Bell,” Pumpelly,” 
Marvine,” Irving,’* Chamberlin,“ and Sweet.’® Rivot’® only has maintained 
1Qp. cit., p. 55. 
2 Joint Documents, Mich., p. 490. 
3Am. J. Sci. (1) XLIX, pp. 62-72. 
4“Geology of the Western and Northwestern Portions of the Valley of Lake Superior,” in Owen’s 
Geological Survey of Wisconsin, Iowa, and Minnesota, pp. 333-424, 
5 Geology of Canada, 1863, pp. 698, 699. 
6Seventh and Ninth Annual Reports of the Geological and Natural History Survey of Minnesota. 
7Geology of Canada, 1863, pp. 71,72. 
8Second Geol. Survey of Penn., Azoic Rocks, Part I, p. 256. 
2Report of Progress of Geol. Survey of Canada, 186366, pp. 115-164. 
10Report of Progress of Geol. Survey of Canada, 1866—69, pp. 313-364, 
Proc. Amer. Acad. 1878, XIII, pp. 253, 254. 
12 Geological Survey of Michigan, 1873, Part II, pp. 109-112. 
18 Geology of Wisconsin, Vol. III, p. 7. 
14Geology of Wisconsin, Vol. III, p. 391. 
15Geology of Wisconsin, Vol. III, p. 336. 
16 Annales des Mines (5), VIII, 173-328, and 364, 374. 
