220  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1906,  PART    I. 
dump  has  been  thoroughly  disintegrated  by  exposure  to  the  weather 
for  thirteen  years.  It  contains  nearly  6  per  cent  more  water  and  less 
silica  and  alumina  than  the  tunnel  rock,  though  the  ratio  of  silica  to 
alumina  is  practically  the  same  in  both.  In  C  the  ratio  of  silica  toil 
alumina  is  lowest  and  the  material  contains  practically  no  iron.  D 
is  highly  siliceous,  as  was  to  be  expected  from  its  appearance  and  its 
exposed  position. 
INCRUSTATIONS. 
Two  forms  of  incrustations  having  entirely  different  chemical 
composition  are  common  in  association  with  the  alum  rock  wherever 
the  conditions  are  favorable  for  their  accumulation. 
Upon  the  sides  of  the  cliffs,  particularly  where  they  overhang  or 
are  undercut  and  pitted  b}T  erosion  and  afford  some  protection  from 
rain,  are  extensive  deposits  of  material,  evidently  leached  out  of  the 
adjacent  rock  and  deposited  from  solution.  These  incrustations 
vary  in  thickness  from  a  few  inches  to  3  or  4  feet.  The  outer  surface 
has  a  fluted  appearance,  resembling  some  stalactitic  cave  deposits. 
It  is  generally  yellowish  white  in  color  and  is  fairly  hard,  with  al 
porous,  cellular  structure.  Within  this  outer  crust  the  material  is 
much  softer  and  in  many  places  occurs  as  a  perfectly  white  powder! 
This  incrustation  consists  of  the  hydrated  sulphate  of  aluminum, 
alunogen.  The  white  powdery  material  is  very  pure  and  has  prac- 
tically  the  theoretical  composition  of  alunogen;  the  outer  crust  con- 
tains a  slight  amount  of  impurities  and  less  than  the  theoretical 
amount  of  water. 
The  following  analyses  give  the  composition  of  this  incrustation: 
Analyses  of  alunogen  from  Gila  River,  New  Mexico. 
A. 
B. 
c. 
A1203 
16.29 
36.93 
46.45 
15.52 
34.43 
7.62 
15.3 
so3 
36.fi 
EsO 
48.7 
99.67 
100.13 
100.0 
A.  Analyst,  George  H.  Corey.    Carefully  selected  crystals. 
B.  Analyst,  F.  W.  Clarke.     Pinkish  crusts.    Bull.  U.  8.  Geol.  Survey  No.  9,  1884,  p.  13. 
C  Theoretical  composition  of  alunogen— Al2(SO<) 3+ 18  H2O. 
The  second  form  of  incrustation  is  most  abundant  on  the  walls  of 
the  tunnels  which  have  been  driven  into  the  alum  rock,  particularly 
in  No.  2.  This  tunnel  was  driven  in  1893,  and  since  that  time  its 
walls  have  become  coated  with  a  heavy  incrustation  from  3  to  6  inches 
thick,  consisting  of  halotrichite,  a  sillsy  fibrous  mineral  closely  resem- 
bling asbestos  in  appearance.  It  has  a  very  pale  greenish  color  and 
a  strongly  astringent  taste.     The  outer  exposed  surface  of  the  incrus- 
