388  LIEUT. -GEN.  C.  A.  McMAHON - NOTES  ON  DARTMOOR.  [Aug.  I  893, 
cuts  obliquely  through  the  bedding  of  the  slates,  the  dip  of  which  is 
here  northerly,  but  near  its  margin  it  sends  numerous  veins  into 
the  slates  and  infolds  large  slabs  of  them  in  its  arms.  The  slates  in 
contact  with  the  dyke  are  highly  altered.  The  intrusive  character 
of  this  rock  has  also  been  recognized  in  recent  years  by  Mr.  Trank 
Rutley,  T.G.S.,  who  has  kindly  furnished  me  with  an  extract  from 
an  unpublished  report  on  it. 
A  brief  reference  to  the  petrological  characters  of  this  rock  will 
be  found  in  Mr.  Teall’s  ‘  British  Petrography  ’  (p.  316).  I  shall 
therefore  content  myself  with  mentioning  an  interesting  character¬ 
istic  of  the  rock  that  has  not  yet  received  notice.  I  have  examined 
live  thin  slices  of  the  Meldon  granite,  and  find  that  a  striking  feature 
in  them  is  that  the  leaves  of  mica  and  the  prisms  of  felspar  are 
sometimes  bent  and  in  some  cases  broken  ;  and  that  the  groundmass 
consists  of  a  mosaic  of  quartz  and  felspar,  which  some  writers  seem 
to  regard  as  proof  of  dynamo-metamorphism.  This  mosaic  penetrates 
into  the  cracks  in  the  felspar,  and  in  some  cases  it  divides  the  ruptured 
parts.  Curiously  enough,  none  of  the  quartz  exhibits  strain-shadows. 
I  see  no  reason  to  suppose  that  this  rock  gives  evidence  of  dynamo- 
metamorphism.  It  is  true  that  some  of  the  epidiorites  in  this 
locality  are  foliated,  but  the  strike  of  their  foliation  is  discordant 
with  the  strike  of  this  dyke.  Moreover,  the  foliated  epidiorites  are 
quite  close  to,  and  this  granite-dyke  is  in  the  vicinity  of,  the  Meldon 
black  Culm  limestone,  regarding  which  Mr.  Ussher  notes  “  the  Culm 
limestones  in  Meldon  quarry  near  Okehampton  can  scarcely  be  said 
to  be  altered  at  all,  although  within  a  mile  of  the  Granite  boundary 
[namely,  the  main  granite-mass  of  Dartmoor]  and  in  the  vicinity  of 
Greenstones  and  hard  flinty  Culm  Measures  ”  (p.  210). 
The  gneissose  granite  of  the  Himalayas  at  Dalhousie,  where  it  is 
8  miles  thick,  is  full  bf  this  ‘tesselated’  mixture  of  quartz  and 
felspar,  but  the  rock  contains  undoubted  inclusions  which  forbid  the 
supposition  that  it  was  ground  down  after  consolidation.  A 
beautiful  photograph  of  one  of  these  (which  speaks  for  itself)  is 
published  in  the  Records  of  the  Geological  Survey  of  India,  vol.  xvii. 
(1884)  p.  174.  Thin  slices  examined  under  the  microscope  show 
that  the  granite  exhibits  this  tesselated  structure  up  to  the  very 
margin  of  the  included  fragment  of  schist  shown  in  the  photograph. 
The  mosaic  structure  is  clearly  not  due,  in  this  case,  to  shearing 
subsequent  to  the  consolidation  of  the  rock,  for  the  included  schist 
has  not  been  sheared. 
Some  of  the  minerals  of  the  Meldon  granite  show  that  they  have 
suffered  from  abrasion  and  pressure ;  that  is  to  say,  some  of  the 
micas  and  felspars,  as  before  stated,  have  been  bent,  cracked,  and 
even  broken  ;  but  these  features,  like  the  dark  stripes  along  the 
lower  side  of  the  vein  on  the  banks  of  the  Lyd  (see  above,  p.  386), 
are,  I  think,  sufficiently  accounted  for  by  supposing  that  they  were 
produced  either  when  the  granite  was  forced  through  the  jaws  of  a 
fissure  in  the  slates,  or  by  strains  while  the  dyke  wras  solidifying. 
Mr.  Ussher  holds  that  a  long-continued  north-and- south  pressure 
was  sufficient  to  4  fuse  ’  the  pre-Devonian  rocks  that  occupied  the  sites 
