732  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
Irving,49  in  1899,  gives  a  detailed  description  of  the  geology  of  an 
area  in  the  northern  Black  Hills  of  South  Dakota.  The  Algonkian 
rocks  consist  of  quartzites,  slates,  phyllites,  and  schists,  all  of  sedi- 
mentary origin.  No  new  point  is  added  concerning  the  stratigraphy 
of  the  pre-Cambrian  rocks. 
Jaggar,50  in  1901,  in  connection  with  the  discussion  of  the  lacco- 
liths of  the  northern  Black  Hills,  incidentally  refers  to  the  structure 
of  the  Algonkian.  Its  lamination  abuts  abruptly  against  the  hard 
basal  Cambrian  quartzite  or  the  conglomerate,  and  has  a  fairly  uni- 
form strike  of  north-northwest.  The  Algonkian  surface  is  seen  to 
be  warped. 
Jaggar,51  in  1904,  describes  the  general  geology  of  the  Black  Hills 
and  gives  particular  attention  to  the  dynamics  of  the  later  intrusions 
of  the  northern  part  of  the  uplift.  The  southern  portion  was  occu- 
pied by  massive  ancient  pegmatite  granites,  themselves  pre-Cambrian 
intrusives  in  Algonkian  strata.  Probably  they  acted  as  a  rigid 
cementing  and  hardening  agent,  to  prevent  fracturing,  in'  the  south- 
ern schists ;  the  northern,  less  indurated  phyllites  cracked  and  faulted 
more  readily  to  permit  the  younger  intrusives  to  rise  from  the  depths. 
The  northern  exposed  schist  areas  contain  many  hundred  dikes  and 
some  stocks;  these  must  have  induced  movements  of  horizontal  ex- 
tension in  the  schist,  and  such  movements  are  attested  by  bedding- 
plane  faults  at  the  base  of  the  Cambrian.  The  dikes  have  a  common 
trend,  and  dip  parallel  with  the  schistosity.  The  dip  gave  them  a 
tendency  to  spread  in  the  Cambrian  in  one  direction  more  readily 
than  in  another. 
Two  illustrative  sections  are  given.  North  of  the  Homestake  mine, 
on  Deadwood  Creek  near  Central,  Algonkian  rocks  appear  as  follows, 
from  west  to  east:  Graphitic  schist,  mica  schist,  heavy  ferruginous 
black  schist  with  quartzite  bands  cut  by  irregular  white  quartz  bodies 
which  form  a  distinct  zone,  ferruginous  schist,  mica  schist,  all  dipping 
toward  the  east;  mica  schist  with  thin  sandstone  stringers,  dipping 
to  the  west.  This  sudden  change  of  dip  just  opposite  the  De  Smet 
and  Homestake  ore  bodies  is  significant,  and  suggests  that  perhaps 
the  great  ore  body  may  fill  a  synclinal  saddle  pitching  to  the  southeast. 
A  section  from  north-northwest  to  south-southeast  along  the  ridge 
northeast  of  the  Clover  Leaf  mine  is:  Garnetiferous  mica  schist, 
graphitic  schist,  ferruginous  quartzite,  amphibolite,  mica  schist,  white 
quartz,  mica  schist,  amphibolite,  quartzite,  and  amphibolite. 
SUMMARY    OF    PRESENT    KNOWLEDGE. 
In  the  Black  Hills  is  a  great  series  of  Algonkian  rocks,  composed 
chiefly  of  mica  slates,  with  quartzites,  schist  conglomerates,  and 
ferruginous  chert,  and  smaller  areas  of  mica  schists  and  mica  gneisses 
of  unknown  but  probably  great  thickness.     They  extend  over  an  oval 
