302  CONTRIBUTIONS    TO   ECONOMIC    GEOLOGY,  1906,  PART    I. 
commonly  known  as  adobe  cla}^  usually  of  residual  origin,  which  is 
almost  universally  used  by  the  Mexican  inhabitants  of  the  region  in 
the  manufacture  of  sun-dried  brick  to  be  used  in  constructing  their 
buildings.  In  many  places  within  the  field  timber  is  scarce  or  entirely 
absent,  and  building  stone  is  not  at  hand.  Consequently  this  adobe 
clay  is  of  great  value  as  a  building  material.  The  Indians  of  the 
Navaho  Reservation  use  it  to  a  large  extent  as  a  mortar  and  plaster 
in  their  stone  huts.  That  the  Aztecs  successfully  used  this  same 
material  as  a  mortar  in  their  cliff  dwellings  and  other  buildings  is 
attested  by  its  presence  in  the  many  ruins  of  these  dwellings  that  are 
now  scattered  over  the  greater  portion  of  the  area.  In  many  of  the 
cliff-house  ruins,  at  places  well  protected  from  the  weather,  this  clay 
mortar  is  evidently  as  firm  and  coherent  as  when  the  houses  were 
built,  centuries  ago. 
