392  LIEUT. -GEN.  C.  A.  MCMAHON— NOTES  ON  DARTMOOR.  [Aug.  1893, 
epidiorites  come  up  to  the  granite,  as  at  White  Tor,  and  are  ap¬ 
parently  cut  off  by  it.  I  looked  for,  hut  failed  to  find,  any  instance 
in  which  the  diorite  cuts  the  granite,  and  no  previous  observer  has 
recorded  a  case  of  this  kind.  On  the  other  hand,  we  have  a  recorded 
case  (noted  by  De  la  Beche  and  admitted  by  Mr.  Ussher,  p.  200) 
in  which  an  "el  van  cuts  one  of  these  ‘  greenstones.’  A  glance  at 
Mr.  Ussher’s  Map  No.  2  shows  how  these  el  vans  radiate  from  the 
granite -masses  (e.  g.  the  Camelford  elvan  from  the  Brown  Willy),- 
and  this  is  so  evident  that  Mr.  ITssher  admits  the  eruptive  character 
of  the  elvans  and  suggests  that  they  “  were  ejected  through  fissures 
in  granite  ”  (p.  199).  Clearly,  then,  the  igneous  magma  from  which . 
the  elvans  were  shot  out  was  situated  at  the  base  of  the  granite- 
masses  from  which  they  radiate. 
The  figure  of  which  King  Nebuchadnezzar  dreamed  had  a  head  of 
gold,  but  feet  partly  of  clay.  Here  the  figure  of  Dartmoor  brought 
before  the  eye  of  the  imagination  by  Mr.  Ussher  is  the  converse  of 
this  conception,  for  it  has  a  metamorphic  top  and.  an  eruptive  basis. 
Tor  my  own  part,  I  think  the  evidence  favours  the  old  view  :  namely, 
that  the  granite  of  Dartmoor  is  as  truly  an  eruptive  rock  as  the  elvans, 
though  some  of  the  latter  may  represent  later  phases  of  the  eruption. 
Another  point  alleged  is  that  there  has  been  a  reciprocal  meta¬ 
morphic  action  between  the  granite  and  the  slates  (pp.  215,  21  < ).  I 
pass  over  the  question  of  schorl  and  its  abundance  along  the  margin 
of  the  granite,  as  I  do  not  think  its  discussion  would  elucidate  any 
point  at  issue  in  the  present  enquiry.  Schorl  is  present  in  every 
slice  of  Dartmoor  Granite  that  I  have  examined,  even  in  a  slice  cut 
off  a  specimen  from  Princetown.  The  normal  type  of  the  Dartmoor 
Granite  is  strongly  porphyritic,  the  porphyritic  crystals  of  felspar 
attaining,  as  at  Princetown,  a  length  of  and  a  breadth  of  3| 
inches.  Along  the  margin  of  the  slates,  on  the  other  hand,  the 
porphyritic  type  usually  disappears  and  the  granite  becomes  compa¬ 
ratively  fine-grained  and  homogeneous  as  to  the  size  of  its  grains. 
An  examination  of  thin  slices  and  a  comparison  between  the  por¬ 
phyritic  and  the  marginal  non-porphyritic  types  have  led  me  to 
believe  that  they  are  both  parts  of  the  same  granite.  Both  contain 
quartz,  felspar,  mica,  schorl,  and  garnet.  The  felspar  is  in  part 
plagioclase,  and  some  of  the  latter  has  the  optical  characteristics  of 
oligoclase.  The  mica  is  principally  a  dark  species,  but  there  is  more 
or  less  of  a  silvery  variety. 
The  best  locality  for  studying  the  two  types  of  granite,  that  I  have 
seen,  is  in  the  valley  of  the  Tavy  in  the  neighbourhood  of  Hill 
Bridge,  between  Hill  Town  and  White  Tor.  The  passage  from  the 
fine,°even-grained  type  to  the  porphyritic  normal  type  is  rapid.  In 
the  bed  of  the  river,  owing  to  a  long  period  of  dry  weather,  I  was 
fortunate  enough  to  reach  a  mass  of  granite  in  situ,  worn  smooth  on 
the  surface  by  the  water,  that  showed  the  actual  blending  of  the 
two  types.  About  50  yards  from  where  the  slates  first  crop  out 
(see  ante)  a  mixture  of  the  porphyritic  and  fine-grained  varieties 
1  Ought  not  this  to  be  ‘  Whit  Tor  ’?  It  is  so  pronounced  by  the  people  of 
the  neighbourhood. 
O  < 
