Roth  mentions l  an  induration  evidently  somewhat  similar  to
that  described  by  Vom  Rath  above,  as  having  taken  place,  on
the  surface  of  a  reddish  yellow  sandstone  in  Fezzan,  North
Africa.  The  crust  thus  formed  was  so  dense  and  hard  as  to
break  with  a  shell-like  fracture  resembling  basalt.  A  similar
incrustation  on  sandstone  from  the  Lydian  desert  was  found  to
consist  of:  manganese  oxide,  30.57%  ;  iron  oxide,  36.86%  ;
alumina,  8.91%  ;  silica,  8.44%  ;  barium  oxide,  4.89%  ;  sul-
phuric acid,  4.06  %  ;  phosphoric  acid,  0.25  %  ;  and  water,  5.90  %.
W.  P.  Blake  has  described  boulders  from  the  Colorado
desert  colored  exteriorly  by  what  he  regarded  as  organic  matter
received  from  water  during  a  period  of  submergence.  Similarly
discolored  quartzitic  boulders  brought  by  G.  K.  Gilbert  from
the  Sevier  desert  in  Utah,  and  examined  by  the  present  writer,
show  a  thin  dark  varnish-like  coating,  not  inaptly  named  by  Mr.
Gilbert  "  desert  varnish,"  and  which  consists  largely  of  oxides
of  iron  and  manganese,  though  a  slight  amount  of  organic
matter  is  present.  In  this  case  the  rock  is  composed  not  wholly
of  quartz  granules,  but  carries  interstitial  calcite  and  feldspathic
granules.  Near  the  discolored  surface  of  the  boulders  these  in-
terstitial calcites  are  found  quite  dissolved  away,  leaving  cavities
stained  by  a  dark  deposit  which  reacts  for  iron  and  manganese.
Inasmuch  as  acid  solutions  obtained  from  fresh  and  uncolored
portions  of  the  boulders  give  faint  reactions  of  the  same  nature,
it  seems  very  probable  that  the  crust  is  due  to  a  concentration
of  these  metals  in  a  condition  of  higher  oxidation  on  the  surface,
whither  they  have  been  brought  by  capillarity,  while  the  more
soluble  lime  carbonate  was  removed.2

1  Allegemeine  u.  Chemische  Geologie,  2d  ed.,  Vol.  Ill,  p.  215.
