Vol.  49.]  LIEUT.-GEN.  C.  A.  McHAHON - NOTES  ON  DARTMOOR.  385 
32.  Notes  on  Dartmoor.  By  Lieut.-General  C.  A.  McMahon, 
E.G.S.  (Bead  May  24th,  1893.) 
The  geology  of  Dartmoor  has  already  furnished  material  for  a  some¬ 
what  extensive  literature;  but,  as  many  points  in  connexion  with 
this  region  are  still  in  dispute,  the  discussion  of  the  subject  cannot 
be  considered  as  finally  closed.  The  view  advocated  by  De  la  Beche 
was  that  the  granite  of  Dartmoor  was  intrusive  in  the  adjoining 
sedimentary  rocks,  and  that  the  metamorphism  exhibited  by  the  Culm 
and  Devonian  beds  for  a  certain  distance  round  the  margin  of  the 
granite  was  due  to  the  contact- action  of  the  heated  eruptive  rock. 
Quite  recently,  however,  this  view  has  been  challenged  by  Mr. 
W.  A.  E.  Ussher,  E.G.S.,  of  the  Geological  Survey,1  and  it  is  alleged 
by  this  observer  that  “  the  genesis  of  the  Devon  and  Cornish  granites 
.  .  .  .  resulted  from  the  metamorphism  in  situ  of  pre-existing  rocks 
of  pre-Devonian  age,  which  had  in  a  rigid  state  exercised  an 
obstructive  influence  on  the  N.  and  S.  movements,  and  had  thereby 
produced  great  mechanical  effects  on  the  surrounding  strata  prior  to 
the  alteration  of  the  latter  ”  (p.  219).  The  result  of  this  dynamic 
action  is  spoken  of  as  ‘  fusion  ’  (pp.  207,  217)  ;  the  “  fusion  and 
subsequent  consolidation  would  appear  to  have  been  effected  in  situ  ” 
(p.  217),  and  “  the  consolidation  of  the  granite  in  its  present  aspect 
may  therefore  be  regarded  as  post-Carboniferous,  or  possibly  coeval 
with  the  later  stages  of  Carboniferous  deposition  ”  (p.  218). 
If  I  understand  Mr.  Ussher  aright,  the  north-and-south  move¬ 
ment  did  not  set  up  any  material  metamorphic  change  in  the  Culm 
and  Devonian  rocks,  but  it  was  sufficient  to  fuse  the  rigid  pre- 
Devonian  rocks  on  which  they  rested ;  and  it  was  the  heat  of  this 
fused  mass  that  produced  the  “  contact-metamorphism  on  the  rocks 
bordering  the  Dartmoor  Granite  ”  (p.  209). 
I  had,  prior  to  my  visit  to  Edinburgh  in  1892,  arranged  to  spend 
a  month  at  Lydford,  on  the  western  flank  of  Dartmoor  ;  and  having 
heard  Mr.  Ussher’s  paper  read  before  the  British  Association,2  in 
which  the  views  more  fully  expressed  in  the  paper  above  quoted 
were  foreshadowed,  I  paid  particular  attention,  during  my  residence 
at  Lydford,  to  an  examination  in  the  field  of  points  connected  with 
the  questions  involved,  and  I  have  since  studied  under  the  micro¬ 
scope  a  good  collection  of  thin  slices  made  from  specimens  then 
collected.  The  object  of  the  present  paper  is  to  place  before  the 
Society  the  results  of  my  observations  so  far  as  they  relate  to  the 
Dartmoor  Granite.  I  am  the  more  disposed  to  do  so  as  the  author 
has  done  me  the  honour  to  quote  from  a  paper  on  the  Culm  Measures 
at  Bude,3  in  which  I  suggested  that  the  materials  of  which  the 
Bude  rocks  are  composed  were  derived  from  a  granite.  That  paper, 
however,  does  not  support  the  contention  which  it  is  at  present  my 
1  ‘  The  British  Culm  Measures/  reprint  from  Proc.  Somerset  Archseol.  &  Nat. 
Hist.  Soc.  vol.  xxxviii.  (1892). 
2  Geol.  Mag.  for  1892,  p.  467.  3  Ibid.  1890,  p.  106. 
