230 '. I;. VAN III-I. — PRE-CAMBRIAN O] IIIK BLACK HILLS 



Upon examining their sections, it turns oul that they vary in character from 

 a somewhat altered uumistakable diorite i" a completely crystalline horn- 

 blende-schist or chlorite-schist While sufficient time was not given to a study 

 of their field relations, no evidence was seen that any of them vary into the 

 mica-elates and mica-schists. They arc all regarded as ancient eruptive rocks 

 which have partaken of the alteration-effects of the forces that metamor- 

 phosed the fragmental series. This being the case, we have- within the Black 

 Hill- pre-Cambrian ana. schists which an- of eruptive ami of clastic origin. 

 In only one or two cases has a hornblende-schist been found, however, which 

 shows any indication of belonging to the clastic series, and in this rock a 

 larger araounl of biotite than hornblende i- present. The foregoing rocks, 

 with t: ption of that just alluded t<». are very similar !•> certain rocks 



of the Lake Superior region in which Dr. George 11. Williams has carefully 

 traced out series running from undoubted eruptives to hornblende-schists ami 

 chlorite-schists My lack of material, as well as limitation in space, pre- 

 vents an attempt t<> do the like with the 15 lack Hills rocks. 1 [owever, it may 

 be said that the thin sections give a tolerably complete gradation from an 

 unmistakable diorite t" one in which the e i> little or no feldspar, quartz and 

 hornblende taking its place, and the rock becoming a crystalline hornblende- 

 Bchist. A further set of alterations has then seized upon certain of them bo 

 that their background contains, in addition to the quartz, a good deal of 

 call \t the same time the hornblende has passed over into chlorite or into 



chlorite and epidote, the rock thus becoming a chlorite-schist or an epidotic 

 chlorite schist. 



The granites have Keen -<> fully described by Newton and Caswell thai I 

 give them little -pace. Tiny are in the main coarse-graiued muscovite- 

 ■ In- (»nly important minerals being muscovite, quartz, ami feldspar, 

 the latter including orthoclase, microcliue, ami plagioclase. 1 hese granites 

 met imes so coarse a- to give muscovite approaching that of a mer- 

 chantable character. These coarse phases are by mi mean- universal : and 

 the) pa-- into rocks which have all the characteristics of muscovite biotite- 

 irdi nary type. A.lso, quite frequently they vary into t<>nr 

 malin i-granite, this miueral l>ein_ r occasionally the only imp irtanl one aside 



in the quartz ami feldspar. 



NaTUKI OP t >i:h.i\ \ I. BED! mi NT. 



va that the detritus from which the Black llill- 



leveloped was almost wholly quartz ami feldspar — 



p< i hap- mingled in place- with so fiue a male rial that it could only he called 



mud. n which quartz ami feldspar are largely 



<-Y " . II 



