Yol.  49.]  LIEUT. -GEN.  C.  A.  McMAHON - NOTES  ON  DARTMOOR.  397 
we  not  find  signs  of  extra  disturbance  for  a  considerable  distance 
outside  that  mass,  at  right  angles  to  the  pressure,  and  a  crushed 
zone  (indicating  one  where  the  pressure  was  inadequate  for  fusion) 
parallel  with  and  inside  of  the  supposed  fused  zone  ?  Was  there 
any  evidence  that  a  rock  could  be  fused  by  pressure  alone,  any 
more  than  by  a  gentle  stewing  in  sea-water,  which  also  had  been 
suggested  ?  Difficulties  no  doubt  there  were  in  accounting  for  the 
relation  of  the  granite-masses  to  the  stratigraphy  of  the  country, 
but  these  were  not  confined  to  granite  or  to  Dartmoor.  No  good 
was  done  for  science  by  proposing  hypotheses  which,  in  avoiding 
one  difficulty,  raised  a  number  of  others  far  more  formidable. 
Prof.  Hull  observed  that  he  fully  concurred  with  the  Author 
that  the  granite  of  Dartmoor  is  intrusive.  He  thought  the  flaggy 
structure  (‘  pseudo-bedding  ’)  in  granite  was  not  at  all  uncommon. 
It  was  very  well  developed  in  the  granite  of  the  Mourne  Mountains, 
which  was  certainly  intrusive,  and  he  regarded  the  structure  as 
originating  in  planes  of  cooling  more  or  less  parallel  to  the  roof  or 
wall  of  the  stratified  masses  into  which  the  molten  matter  had  been 
intruded.  This  was  the  case  in  the  granite  of  Mourne,  and  had  its 
counterpart  in  the  platy  structure  not  uncommon  in  dykes  of  trap, 
where  the  planes  were  developed  parallel  to  the  walls  of  the  dyke. 
The  Author,  in  reply  to  Mr.  Watts,  said  that  the  word  used  by 
Mr.  ITssher  was  ‘  fusion/  and  it  was  applied  to  the  results  of  the 
N.  and  S.  squeeze  on  the  rigid  and  obstructing  pre-Devonian  rocks. 
The  eruptive  origin  of  the  elvans  was  admitted  by  Mr.  Ussher,  and 
they  were  said  to  have  appeared  subsequently  to  the  consolidation  of 
the  granite.  With  reference  to  Mr.  Teall’s  remarks,  the  Author 
stated  that  he  admitted  that  a  4  tesselated  ’  structure  might  result 
from  dynamo-metamorphism ;  he  was  only  concerned  to  show  that 
this  structure  did  not  necessarily  denote  dynamo-metamorphism, 
and  that  it  was,  not  unfrequently,  produced  in  other  ways.  He 
had  not  attempted  a  general  criticism  on  Mr.  Ussher’s  paper,  and 
had  rigidly  restricted  himself  to  points  regarding  which  his  own 
observations  in  the  field  traversed  Mr.  Ussher’s  case.  In  answer  to 
Prof.  Bonney,  he  was  of  opinion  that  the  exposures  of  bare  granite 
were  being  worn  away  by  subaerial  agencies  at  a  more  rapid  rate 
than  the  rounded  grassy  hills  on  which  they  rested.  The  bare 
granite  was  broken  up  by  cross  joints;  the  detachment  of  blocks 
was  continually  going  on,  and  the  hill-sides  were  covered  with 
boulders.  In  reply  to  Prof.  Hull,  the  Author  explained  that  the 
dip  of  the  pseudo-bedding  coincided  with  the  slope  of  the  existing 
rounded  knolls,  or  hills,  of  which  there  were  hundreds,  and  these 
quaquaversal  dips  did  not  coincide  with  what  they  might  assume 
to  have  been  the  original  surface  of  the  granite,  especially  as  the 
granite  probably  rose  in  a  more  or  less  cone-like  form  to  some 
distance  above  its  present  surface. 
