502  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,   1903.  [bull. 225. 
The  smaller  area  above  noted,  north  of  the  principal  deposit,  shows 
sulphur  in  crevices,  in  moderately  large,  perfect  crystals,  nearly  or 
quite  isolated. 
When  exposed  to  the  air  the  alum  rapidly  dehydrates  and  crumbles 
to  a  white  powder,  so  that  it  is  not  conspicuous  in  the  outcrop,  and 
the  real  amount  of  it  present  is  visible  only  when  it  has  been  freshly 
taken  out. 
Manner  of  formation. — These  areas  are  evidently  pipes  or  chimneys 
through  which  sulphurous  volcanic  gases  have  ascended.  Since  the 
rhyolite  bodies  are  probably  intrusive,  the  gases  seem  to  have  directly 
followed  the  intrusion.  This  action  is  familiar  around  recently  active 
volcanoes,  and  is  called  solfataric  action,  from  the  fact  that  sulphur 
(Italian  solfo)  is  deposited  by  it.  For  this  reason  the  volcanoes  of 
Italy,  Mexico,  and  other  places  yield  a  large  amount  of  the  world's 
sulphur.  The  formation  of  alum  by  the  escaping  steam  and  gases  of 
these  solfataras  is  also  known  to  occur  in  many  localities.  While  the 
sulphur  is  a  direct  sublimate  from  the  sulphurous  gases,  as  its  occur- 
rence just  described  in  the  Silver  Peak  deposits  indicates,  the  alum, 
which  is  a  hydrous  sulphate  of  aluminum  and  potassium,  is  formed  by 
a  combination  of  the  steam  and  the  sulphuric  acid  emitted  from  the 
solfataras  with  the  potash  and  aluminum  contained  in  the  rhyolite. 
This  combination  is  rendered  possible  by  a  preliminary  decomposition 
of  the  rhyolite  by  the  escaping  gases.  The  presence  of  cinnabar  is 
also  interesting,  since  this  mineral  is  one  of  those  which  has  been  found 
as  a  sublimate  on  the  walls  of  crevices  in  volcanoes,  as,  for  example, 
at  Vesuvius,  where  it  has  been  deposited  by  jets  of  escaping  gases. 
The  deposit  of  cinnabar  at  Steamboat  Springs,  some  distance  north  of 
here  and  just  north  of  Carson,  is  also  significant. 
Commercial  aspects. — Though  the  alum  in  these  prospects  is  present 
in  far  larger  quantity  than  the  sulphur,  it  is  somewhat  more  localized. 
It  forms  an  irregular  network  of  veinlets,  and  yet  from  the  manner 
of  formation  the  chimney  undoubtedly  continues  downward.  The 
decomposed  rhyolite  is  so  friable  that  the  material  could  easily  be 
worked  on  a  large  scale.  The  rhyolite  itself,  in  the  alum  locality,  has 
been  found  b}^  analysis  to  contain  a  large  percentage  of  alum.  The 
whole  deposit,  therefore,  would  have  to  be  worked  together,  and  the 
sulphur  could  also  be  collected  as  a  by-product. 
