396  LIEUT. -GEN.  C.  A.  McMAHON - NOTES  ON  DAETMOOE.  [Aug.  1 893, 
Mr.  Teall,  continuing,  said  he  was  sure  the  President  would 
agree*  with  him  that  Mr.  Ussher’s  communication  contained  a 
record  of  many  valuable  facts.  There  was  at  present  no  really 
satisfactory  explanation  of  the  structural  relations  of  the  granite  to 
the  surrounding  rocks.  Realizing  the  difficulties  which  beset  the 
current  theories,  Mr.  Ussher  had  been  led  to  propose  the  one  under 
discussion,  which,  however,  he  (the  speaker)  could  not  accept. 
He  considered  that  ‘  tesselated  ’  structures  arose  in  consequence  of 
dynamo-metamorphism,  but  he  agreed  with  General  McMahon  that 
they  might  be  formed  in  other  ways.  #  #  . 
Mr.  Rutley  expressed  his  inability  to  offer  any  unbiassed  criti¬ 
cism  upon  this  paper,  because,  in  the  first  place,  he  entertained  a 
profound  regard  for  the  opinions  of  Sir  Henry  He  la  Beche ;  in  the 
next  he  was  strongly  impressed,  from  personal  experience,  by  the 
extreme  accuracv  of  Mr.  Ussher’s  field-work,  although  with  regard 
to  his  theory  he  felt  somewhat  less  confidence.  Last,  but  not  least, 
he  realized  the  importance  of  General  MpMahon’s.  observations. 
So  far  as  his  experience  went,  the  contact  of  granite  with  other 
rocks  usually  showed  a  sharp  line  of  demarcation.  The  small 
felspars  in  some  of  the  lavas  of  the  Brent  Tor  area  weie  at  times 
represented  bv  pseudomorphs  having  a  mosaic  or  tesselated  struc¬ 
ture.  Some  of  the  fragments  in  the  Brent  Tor  tuffs  which  he  had 
formerly  spoken  of  as  pumice  he  now  regarded  as  vesicular  b.asalt- 
glass,  rendered  black  by  separation  of  magnetite.  He.  considered 
the  Meldon  dyke  as  an  apophysis  of  the  Dartmoor  Granite.  So  far 
as  the  origin  of  the  latter  was  concerned,  he  ventured,  to  suggest 
that  a  cauldron  of  molten  matter  might  have  existed  for  a 
lengthened  period  beneath  the  Devonian  and  Cornish  areas,  and 
that  it  showed  its  existence  first  by  eruptions  of  basic  lavas ;  that 
towards,  or  after,  the  close  of  the  Carboniferous  period  the  granite 
of  Dartmoor  might  have  been  the  source  from  which  traehytic  lavas, 
long  since  removed  by  denudation,  might  have  emanated,  having 
reached  the  surface  through  a  great  thickness  of  overlying  rock, 
now  also  removed  by  denudation ;  and  that  from  the  same  reservoir 
the  later  basic  lavas  on  the  east  of  Dartmoor  might  also  have  been 
derived. 
Prof.  Bonney  said  that  he  felt  some  difficulty  in  accepting  the 
Author’s  explanation  of  the  bedded  structure  in  the  granite,  for.it 
appeared  to  be  related  to  the  present  surface  of  the  ground  .5  this, 
however,  only  represented  a  phase  in  the  denudation  of  the  district, 
and  it  might  be  expected  that  the  structure  would  be  produced 
during  an  earlier  one.  In  all  other  points  he  quite  agreed  with 
the  Author.  He  had  seen  streaked  structures,  as  described,  in  more 
than  one  dyke.  Undoubtedly  there  had  been  a  tendency  among 
certain  geologists  to  appeal  to  ‘  tesselated  structure  as  evidence  of 
dynamo-metamorphism.  It  could  be  so  caused,  but  there  were 
varieties  of  this  structure,  produced  by  different  causes.,  four .  of 
which  he  mentioned,  and  generally  distinguishable.  In  his  opinion 
Mr.  TJssher’s  theory  was  quite  untenable.  If  the  fusion  of  a  peri¬ 
pheral  portion  of  the  Dartmoor  mass  was  due  to  crushing,  why  did 
