390  LIEUT.- GEX.  C.  A.  McMAHO^ — NOTES  OX  DARTMOOR.  [A Eg,  1 893, 
Down  formed  a  part,  and  then  the  squeeze  would  surely  have  come 
upon  the  soft  rocks  ? 
This  subterranean  connexion,  however,  which  is  supposed  to  have 
saved  the  zone  of  Devonian  rocks  to  the  east  of  Brown  Willy,  is  not 
said  by  Mr.  Ussher  to  have  existed  under  the  Launceston-Lydford 
zone  of  Lower  Culm  Measures.  What  saved  them?  The  only 
answer  to  this  question  that  I  have  come  across  is  the  following 
reference  to  Mr.  Butley’s  memoir  on  the  4  Eruptive  Bucks  of  Brent 
Tor  :  ’ — 44  Mr.  Butley,  in  commenting  on  the  schistose  1  character  of 
the  volcanic  ash-beds  of  Tavistock  and  round  Brent  Tor,  observed 
that,  so  far  as  dips  can  be  noted,  the  rocks  have  undergone  but  little 
pressure  laterally.  The  abnormal  extension  of  the  Upper  Devonian 
between  the  Dartmoor  and  Brown  Willy  granites  would  show  that 
the  beds  have  been  kept  at  the  surface,  whilst  at  the  same  time 
exposed  to  a  considerable  strain,  effecting  contortions  on  so  small  a 
scale  or  under  such  conditions  as  to  produce  the  effect  of  a  general 
almost  horizontal  bedding  ”  (p.  187). 
The  explanation  of  the  difficulty  which  appears  to  be  suggested 
in  the  remarks  above  quoted  is  that  the  Culm  Measures  were  not 
metamorphosed  by  the  squeeze  that  fused  the  rigid  pre-Devonian 
rocks  into  granite  because  they  were  under  surface- conditions — they 
were  4  kept  at  the  surface.’  This  explanation  will  not,  I  think, 
bear  examination.  From  the  fact  that  the  Brown  Willy,  Kit  Hill, 
Hingston  Down,  and  Dartmoor  masses  consolidated  as  granite,  we 
know  that  they  must  have  consolidated  under  plutonic  conditions  at 
a  considerable  distance  below  the  surface  of  the  earth.  It  seems 
physically  impossible,  therefore,  that  beds  only  a  mile  or  a  mile  and 
a  half  distant  from  these  masses  can  have  been  ‘  at  the  surface.’ 
Again,  Mr.  Ussher  says  that  his  granite  consolidated  4  in  situ  ’ ; 
namely,  in  the  exact  spot  where  the  pre-Devonian  rock  was  squeezed 
into  a  condition  of  fusion.  Well,  if  that  be  so,  at  least  20,000  feet 
of  strata  must  have  been  piled  up  on  the  top  of  the  fused  mass,  and 
the  pressure  of  the  superposed  strata  would  surely  have  forced  so 
much  of  the  granite  as  was  in  a  fused  condition  through  the  Culm 
Beds  lying  4  at  the  surface  ’  within  a  mile,  or  a  mile  and  a  half,  of 
the  granite. 
If  the  north-and-south  squeeze  were  capable  of  fusing  the. whole 
of  a  rigid  rock  covering  an  area  of  225  square  miles,  even  if  the 
fusion  were  accomplished  by  sections — a  supposition  which  my  mind 
can  hardly  grasp — it  ought  to  have  exerted  a  tremendous  effect  on 
the  rocks  in  actual  contact  with  the  granite.  Contortion  exists,  but 
it  is  local ;  as  a  general  rule  the  beds  lie  placidly  against  the  granite 
with  a  very  moderate  dip.  I  have  never  seen  anything  at  all  like 
a  passage  between  the  Culm  or  Devonian  rocks  and  the  granite. 
Whenever  the  granite  and  sedimentaries  are  seen  in  contact,  the 
line  of  division  is  sharp,  both  in  hand-specimens  and  in  the  slices 
examined  under  the  microscope. 
1  ‘  Fissile  ’  would  be  a  less  misleading  term.  These  rocks  are  not  schists  in 
a  strictly  technical  sense. 
