482  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.  [bill.  260. 
This  is  the  only  analysis  available,  and  it  is  remarkable  for  showing 
considerable  variation  from  typical  natural  gas.  The  methane  is  low 
and  the  ethane  and  nitrogen  are  correspondingly  high.    . 
For  nineteen  months  Salt  Lake  City  was  supplied  from  this  source, 
but  the  amount  produced  is  not  definitely  known.  Measurements  at 
the  wells  show  that  an  average  monthly  supply  of  8,500,000  cubic 
feet  was  maintained,  while  measurements  in  the  city  recorded  a  little 
less  than  7,000,000  cubic  feet.  Part  of  this  discrepancy  undoubtedly 
is  due  to  leakage  from  the  delivery  pipe,  and  differences  in  accuracy 
of  measurement  may  account  for  the  rest.  From  September,  1895, 
to  March,  1897,  approximately  150,000,000  cubic  feet  were  supplied. 
The  maximum  amount  was  delivered  in  August,  1890,  when  over 
17,000,000  cubic  feet  were  recorded  at  the  wells. 
From  the  beginning  trouble  was  caused  by  the  gas  coming  spas- 
modically. Each  well,  after  being  opened,  diminished  in  pressure 
until  the  flow  almost  ceased,  but  when  cleaned  out  the  original  pres- 
sures are  reported  to  have  been  attained  again,  the  wells  having 
become  clogged  with  sand  and  clay.  Finally,  however,  in  spite  of  the 
fact  that  new  wells  were  sunk,  the  supply  decreased  until  it  became 
insufficient  for  the  city's  needs  and  the  held  was  abandoned. 
At  present  the  sites  of  a  number  of  the  wells  are  marked  by  Ioav 
mounds  a  foot  or  two  in  diameter,  caused  probably  by  mud  brought 
from  the  Avells  by  artesian  water,  (las  which  can  be  readily  lighted 
commonly  escapes  from  these  mounds,  and  has  been  thus  escaping 
since  the  wells  were  abandoned. 
Whatever  its  origin,  the  gas  appears  to  have  become  imprisoned 
in  lenses  of  sand  which  are  overlain  by  impervious  caps  of  clay. 
The  source  may  possibly  be  in  Tertiary  beds  that  are  thought  to 
underlie  the  Bonneville  deposits.  The  existence  of  such  beds  is 
inferred  from  the  geologic  history  of  the  region,  though  their  char- 
acter, extent,  and  depth  below  the  surface  are  unknown.  It  is  well 
known  that  the  hydrocarbons  which  are  found  east  of  the  Wasatch 
Mountains  occur  in  rocks  of  Tertiary  age.  But  a  more  likely  source 
is  suggested  by  the  fact  that  in  a  number  of  wells  considerable 
amounts  of  organic  matter,  largely  of  vegetable  origin,  have  been 
encountered.  This  organic  matter  was  entombed  in  the  old  lake 
deposits,  and  its  decomposition  may  account  for  the  origin  of  the  gas. 
Though  the  field  described  has  been  the  most  productive  of  several 
known  areas  in  the  Bonneville  region,  it  is  quite  possible  that  similar 
accumulations  will  be  found  in  the  future.  There  is  little  reason, 
however,  for  expecting  much  better  results  than  already  obtained. 
Doubtless  considerable  quantities  of  hydrocarbons  have  been  formed 
from  the  decomposition  of  disseminated  organic  matter  contained  in 
the  lake  beds,  but  much  may   have  escaped.     The  preservation  of 
