294 S. PAIGE ORIGIX OF THE HOMESTAKE ORE BODY 



These pre-Cambrian rocks in the extreme northern part of the hills 

 aie only partially exposed. They occupy an elongate area 8 or 9 miles 

 in length, lying in a northwest-southeast position. The area is sur- 

 rounded and partly covered by Paleozoic sediments; also it contains many 

 late intrusive rocks, which break the continuity of the schists and ob- 

 scure structural relations. 



Bocks 



The pre-Cambrian rocks comprise quartz-biotite schists, garnetiferous 

 schists, slates, calcareous slates, limestones or dolomites, conglomerates 

 and quartzites, and a variety of intermediate types depending for their 

 character on slight original differences in composition. The whole is the 

 metamorphic equivalent of a sedimentary series of no mean proportions. 



These rocks are. in broad terms, distributed in two divergent groups 

 (see sketch map, figure 1, page 295).' The one, an eastern group, strik- 

 ing slightly east of north, comprises from east to west, and, so far as 

 dips are concerned, from top to bottom, biotite schist, with thin bands 

 of slate, a quartzite, garnetiferous schist, a quartzite, slates, a quartzite 

 conglomerate ("upper" 7 conglomerate), a band of slate, a quartzite con- 

 glomerate ("lower" conglomerate), and last a calcareous series grading 

 downward into black slate. The other, the western group, striking west 

 of north, comprises from east to west, first, an alternating series of black 

 slates and quartzites, and next a thick series of slates and phyllites con- 

 taining some thin calcareous beds and some quartzites, and to the west 

 some dominantly quartzitic beds. 



The divergent strike of these two somewhat similar groups of rocks is 

 explained by a fault-line which passes about north 20° to 30° west 

 through the town of Lead, and which, though its precise location is with 

 difficulty determined, is an actual boundary between the two groups. 



Structure 



There is a general structure which prevails over this entire region, 

 namely : The rocks are closely folded and have steep isoclinal dip to the 

 eastward. 2 As stated, however, in the last paragraph, a strong fault 

 divides the region into two structurally opposed parts. 



It is the strike of the axes of the major folds which determines the 

 trend of the rocks within these parts. Minor folds imposed on the limbs 

 of the great folds create strikes at variance with the major strike, but so 

 closely are the rocks appressed that the strike of these minor folds is 

 seldom greatly different from the strike of the major folds. The major 

 folds and also the minor folds, so far as determined, pitch southward. 



2 There are exceptions in relatively small areas. 



