TIN    IN    TEXAS. 
147 
>arts  of  the  range  from  20°  to  45°.  No  sediments  of  early  Mesozoic  age  are  present,  but 
Cetaceous  strata,  including  the  Washita  and  Benton  groups,  occur  in  isolated  areas  west, 
tnd  south  of  the  range.  The  adjacent  lowlands  are  underlain  By  gravel,  sand,  and  ela\  to 
i  depth  of  over  2,000  feet, 
A  considerable  amount  of  granite  occurs  in  the  Franklin  Mountains  along  the  eastern 
>ase,  chiefly  associated  with  lines  of  faulting.     The  granite  is  a  post-Paleozoic  intrusion 
NEW    MEXICO 
Fig.  7. — Geologic  map  of  the  Franklin  Mountains,  Texas. 
id  is  important  as  the  rock  in  which  the  tin  ore  occurs.  It  is  a  medium  to  coarse  tex- 
ired  variety,  superficially  reddish  in  color,  and  weathers  readily  to  a  coarse  sand.  Fresh 
cposures  are  rare  but  from  the  tin  prospects  specimens  are  obtainable,  a  partial  analysis 
;  one  of  which  by  E.  C.  Sullivan,  of  the  United  States  Geological  Survey,  gave: 
Partial  analysis  of  granite  from  tin  prospect  in  Franklin  Mountains,  Texas. 
Percent. 
Si02 73. 76 
CaO 81 
K20 5.  06 
Na20 3. 64 
The  alkalies  are  slightly  higher  and  the  lime  lower  than  in  normal  granite.     The  rock  is 
)mposed  of  preponderating  quartz  and  feldspar,  the  latter  in  excess,  with  subordinate 
