Yol.  49.]  LIEUT. -GEN.  C.  A.  McHAHON NOTES  ON  DARTMOOR.  387 
composition  with  the  granite  of  the  main  mass,  and  other  circum¬ 
stances,  I  cannot  doubt  their  being  apophyses  from  it.  The  granite 
of  the  main  mass  is  apparently,  in  this  locality,  porphyritic  up  to 
its  contact  with  the  slates,  and  these  intrusive  veins  are  also 
porphyritic.  I  saw  no  trace  of  elvans  in  the  slates  in  this  locality, 
or  of  granite-dykes  in  the  granite,  or  of  granite-veins  in  the  Culm 
series  at  a  distance  from  the  main  mass  of  the  granite  ;  and  I  am 
not  aware  of  any  fact  to  support  the  contention  (which  has  not  yet 
been  made)  that  these  intrusive  veins  are  of  later  date  than  the 
granite  of  Dartmoor. 
Mr.  TJssher  does  not  allude  to  the  observations  of  De  la  Beche 
regarding  eruptive  apophyses  emanating  from  the  main  mass  of  the 
granite.1  It  is  possible,  however,  that  he  may  admit  the  correct¬ 
ness  of  these  observations,  but  explain  them  away  in  his  own  mind 
by  supposing  that,  when  his  metamorphic  mass  was  in  a  ‘  fused  ’ 
condition,  pressure  acting  on  the  liquid  mass  forced  portions  of  it 
into  the  adjoining  strata;  but  it  is  difficult  to  reconcile  this  supposi¬ 
titious  theory  with  such  statements  as  the  following :  “  I  think  it 
may  be  admitted  that  the  fusion  and  subsequent  consolidation  ex¬ 
tended  over  a  long  period,  so  that  at  no  one  time  was  any  of  the 
masses  in  a  general  state  of  fluidity,  for  had  this  been  the  case,  as  we 
have  no  reason  to  infer  that  the  N.  and  S.  movements  had  ceased 
to  act,  the  plastic  masses  would  not  have  consolidated  in  the  moulds 
of  their  original  rigid  outlines  ”  (p.  218).  The  Dartmoor  Granite  did 
not,  therefore,  according  to  this  passage,  act  as  an  eruptive  rock. 
I  may  mention  in  this  connexion  that,  in  the  Diver  Tavy,  nearly 
a  quarter  of  a  mile  above  Hill  Bridge,  about  50  yards  of  indurated 
slates  follow  the  granite  in  the  bed  of  the  stream,  and  then  a  reef 
of  granite  8  or  10  yards  wide  cuts  across  the  river-bed,  followed  by 
more  slates.  This  second  outcrop  of  granite  is  certainly  in  situ ,  and 
it  appears  to  be  an  intrusive  sheet.  It  is  here  a  non-porphyritic 
rock  containing  many  garnets,  and  is  of  a  character  exactly  similar 
to  the  granite  along  the  margin  of  the  main  mass,  exposed  higher  up 
the  stream,  which  will  be  alluded  to  farther  on. 
It  is  desirable  also  to  refer  here  to  a  dyke  of  granite  at  Meldon,  on 
the  West  Okements  Diver,  near  the  railway- viaduct  over  the  stream. 
Two  oval  patches  are  marked  on  the  Geological  Survey  map,  but  I 
could  not  find  the  second  outcrop  placed  on  the  map  under  South 
Down.  Much  of  the  mapping  in  this  part  of  Devonshire  seems  to 
have  been  filled  in  from  surface-boulders  and  stray  stones  in  walls, 
and  it  would  not  be  safe,  in  many  cases,  to  draw  any  inference  from 
the  outlines  of  igneous  rocks  shown  on  the  map.  Dor  instance,  the 
outcrop  near  the  viaduct  is  not  oval  in  shape,  as  represented  on  the 
map.  On  the  contrary,  it  is  a  long,  straight  dyke,  striking  about 
W.S.W.  10°  W.  Its  thickness  I  should  estimate  at  about  40  feet. 
At  the  time  of  my  visit  this  granite  was  being  worked  for  road- 
material,  and  the  lie  of  the  dyke  could  be  clearly  seen.  There  is 
no  doubt  about  the  intrusive  character  of  this  rock.  It  not  only 
1  ‘Deport  on  the  Geology  of  Cornwall,  Devon,  and  West  Somerset,’  1839, 
p.  165. 
