386  LIET7T.-GEN.  C.  A.  McMAHON - NOTES  ON  DAETHOOE.  [Allg.  1 893, 
desire  to  controvert.  I  did  not  point  to  the  granite  now  exposed  at 
Dartmoor  as  the  parent  of  the  Culm  rocks ;  and  it  is  obvious  that 
granite  which,  even  according  to  Mr.  Ussher’s  view,  consolidated  at 
the  close  of  the  Carboniferous  period,  or  in  post-Carboniferous  times, 
cannot  have  supplied  by  its  subaerial  waste  material  for  the  Culm 
Measures  at  Dude,  which  are  indicated  in  Mr.  Ussher’s  map  as  of 
Middle  Culm  age. 
The  first  point  to  which  I  directed  my  attention  last  autumn  was 
whether  De  la  Beche  was  correct  in  asserting  that  the  granite  sent 
off  intrusive  veins  from  the  main  mass  into  the  rocks  adjoining  it. 
This  point  is  not  so  easily  decided  as  might  have  been  expected. 
As  a  general  rule,  along  the  western  borders  of  Dartmoor  the  granite 
occupies  higher  ground  than  the  sedimentary  rocks,  and  the  actual 
line  of  contact  is  obscured  by  a  fringe  of  granite-taZ?,^,1  or  by  vege¬ 
table  growth.  In  its  upper  course,  however,  to  the  N.E.  of  the  village 
of  Lydford,  at  the  place  marked  ‘Mary  Emma’  on  the  one-inch 
Ordnance  map,  the  Lyd  is  approximately  the  boundary  of  the  granite 
and  of  the  slates.  The  granite  slopes  down  rapidly  to  the  stream, 
and  the  slates  rise  abruptly  on  the  other  side.  The  actual  boundary 
of  the  granite  appears  to  be  very  close  to  the  left  margin  of  the 
narrow  stream,  and  in  some  cases  to  pass  over  to  the  right  bank. 
A  careful  examination  of  this  locality  revealed  to  me  three  intru¬ 
sive  veins  of  granite,  a  few  inches  thick,  cutting  through  the 
Culm  beds.  They  had  all  the  appearance  of  coming  from  the  main 
mass  of  the  granite.  About  100  feet  in  elevation  above  this  spot, 
and  distant,  as  the  crow  flies,  about  150  yards  from  the  stream,  a 
railway  for  peat  has  been  made  round  the  flank  of  the  hill,  and  I 
here  found  a  granite-dyke  about  1  foot  wide  cutting  obliquely 
across  the  Culm  Measures.  The  granite  in  this  dyke  is  of  medium¬ 
sized  grain  and  contains  porphyritic  felspars  1  inch  in  length.  It 
is  streaked  with  dark  lines  along  its  lower  margin.  The  Culm 
beds  in  which  these  four  granite-veins  occur  are  not  contorted;  they 
dip  about  25°  N.W.,  and  the  cleavage  coincides  with  the  dip. 
A  microscopical  examination  of  four  slices  taken  from  three 
of  these  veins  does  not  enable  me  to  differentiate  the  granite  in 
them  from  that  of  the  main  mass.  These  slices  contain  quartz, 
felspar  (orthoclase  and  plagioclase),  biotite,  and  silvery  mica,  much 
schorl,  some  garnet,  and  a  little  zircon.  The  veins  must  have  been 
injected  under  considerable  pressure  and  heat,  for  the  quartz  is  full 
of  liquid  and  gas  cavities,  and  many  of  the  former  contain  mineral 
deposits  (a  cube  is  a  common  form)  as  well  as  bubbles  ;  and  the 
mineral  deposits  and  bubbles  are  of  large  size  compared  with  the 
area  of  the  cavities  containing  them,  showing  that  the  liquid  had, 
at  the  time  of  injection,  great  solvent  power. 
These  veins  are  undoubtedly  intrusive  veins,  and  although,  owing 
to  the  intervention  of  loose  boulders  and  vegetation,  they  cannot 
actually  be  traced  back  into  the  main  mass  of  the  granite,  yet  they 
are  very  close  to  its  boundary ;  and  from  their  agreement  in  mineral 
1  Now  in  the  form  of  weathered  boulders. 
