Yol.  49.]  LIEUT. -GEN'.  C.  A.  McMAUON - NOTES  ON  DARTMOOR.  389 
of  Dartmoor,  Kingston  Down,  and  Brown  Willy.  He  considers  that 
the  44  fusion”  of  the  “  obstructing  masses  ”  ‘‘commenced  at,  roughly 
speaking,  the  same  period,”  but  the  “  fusion  and  subsequent  consoli¬ 
dation  extended  over  a  long  period,  so  that  at  no  one  time  was  any 
of  the  masses  in  a  general  state  of  fluidity  ”  (p.  218).  It  must  have 
been  a  considerable  squeeze  if  it  were  capable  of  fusing  rocks 
occupying,  in  the  case  of  Dartmoor,  an  area  of  “  about  225  square 
miles,”  and  in  the  case  of  Brown  Willy  of  “  65  square  miles  ” ;  and 
I  think  one  may  fairly  ask  how  such  extraordinary  dynamic  force 
could  have  been  exerted  on  the  rocks  of  this  region  and  have  left  the 
Culm  beds  unaltered.1  Mr.  TJssher  does  not,  apparently,  attribute 
the  metamorphic  changes  set  up  in  the  slates  that  fringe  the  granite 
to  dynamic  action.  He  speaks  of  these  changes  as  the  “  effects  of 
contact-metamorphism”  (p.  209),  and  from  the  general  tenour  of 
his  remarks  at  pp.  209,  210,  I  understand  him  to  use  the  terms 
4 contact-metamorphism ’  and  ‘contact-alteration’  in  their  ordinary 
sense,  and  to  imply  the  thermal  and  hydrothermal  action  of  the 
4  fused  ’  pre-Devonian  rocks  on  the  beds  resting  on  them. 
There  certainly  seems  no  escape  from  the  conclusion  that  the 
metamorphism  of  the  fringing-zone  is  due  to  the  thermal  contact- 
action  of  an  uncooled,  unconsolidated  granite.  The  fringing-rocks 
exhibit  in  the  mineral  changes  set  up  in  them,2  such  as  the  produc¬ 
tion  of  chiastolite  in  beds  rich  in  carbonaceous  material,  evidence  of 
the  contact-action  of  heated  granite.  We  see  these  changes  set  up 
in  other  regions  in  rocks  in  contact  with  eruptive  granite,  and  we 
note  that  they  die  out  as  we  recede  from  the  igneous  rock,  under 
conditions  where  dynamo-metamorphism  is  not  in  question.  There 
are  no  grounds,  that  I  am  aware  of,  for  attributing  the  meta¬ 
morphism  of  the  fringing-zone  of  Dartmoor  to  any  other  cause.  A 
glance  at  the  Geological  Survey  map  of  Devonshire  will  show  that 
the  metamorphism  clings  to  the  granite  and  follows  all  the  remark¬ 
able  sinuosities  of  its  boundary.  How  the  material  point  is  that 
directly  we  get  beyond  this  fringing  metamorphic  zone — that  is  to 
say,  when  we  get  a  mile,  or  a  mile  and  a  half,  from  the  boundary 
of  the  Granite  and  the  Culm  Measures — we  pass  into  unaltered  rocks. 
Is  it  possible  that  a  stupendous  north- an d-south  squeeze  exerted  on 
the  whole  region,  and  capable  of  fusing  a  rigid  rock  covering  an 
area  of  225  square  miles,  would  have  left  these  beds  untouched? 
Mr.  Ussher  remarks  at  p.  192 : — 44  The  intervention  of  the  Hingston 
Down  and  Kit  Hill  granites  renders  it  extremely  probable  that  a 
subterranean  connection  of  the  Dartmoor  and  Brown  Willy  granites 
exists  at  depths  sufficient  to  protect  the  overlying  Devonian  from 
yielding  to  the  compression  to  which  a  greater  thickness  of  soft 
strata  would  be  subjected  by  the  north-and-south  movements.” 
But  a  stupendous  squeeze  capable  of  fusing  the  rocks  of  Dartmoor, 
Hingston  Down,  Kit  Hill,  and  Brown  Willy  must  inevitably  have 
fused  the  44  subterranean  connection  ”  of  which  Kit  Hill  and  Hingston 
1  I  think  the  condition  of  these  beds  is  fairly  comparable  with  those  at  Bude, 
as  to  which  see  my  paper  in  Geol.  Mag.  for  1890,  p.  106. 
2  These  have  already  been  described  by  previous  writers. 
