Vol.  49.]  LIEUT. -GEN.  C.  A.  MciIAHON — NOTES  GN  DARTJIOOR.  395 
little  by  little  grown  into  deep  joints,  and  so  the  pseudo-bedding 
has  been  gradually  produced.1  With  the  dominant  pseudo-bedding 
other  joints,  roughly  speaking  at  right  angles  to  the  bedding,  have 
also  been  formed  by  the  constantly-recurring  expansion  and  con¬ 
traction.  A  similar  phenomenon  from  a  like  cause  may  be  commonly 
seen  in  surface-pits,  in  various  parts  of  England,  when  solid  strata 
are  brought  near  the  surface. 
Discussion. 
The  President  remarked  that,  as  the  paper  alluded  to  was  not  in 
evidence  nor  its  Author  present,  the  discussion  might  be  less 
effective,  notwithstanding  that  the  points  actually  criticized  had 
been  so  well  brought  out  by  General  McMahon. 
Dartmoor  had  been  the  subject  of  many  an  hypothesis.  A  few 
years  ago  Ussher  himself  maintained  that  it  was  a  laccolite  ;  Worth 
saw  in  it  the  hypogene  condition  of  a  great  Devonshire  volcano  ; 
Hunt  regarded  it  as  an  Archaean  massif.  There  has  always  existed 
a  school  who  have  tried  to  prove  that  certain  granites  are  due  to 
some  form  of  metamorphism.  It  had  been  reserved  for  Mr.  Ussher 
to  invent  a  modified  metamorphic  hypothesis.  He  is  too  ingenious 
to  maintain,  like  the  old  metamorphists,  that  a  potash  granite  is  the 
result  of  the  metamorphism  of  the  encasing  rocks  :  the  impossibility 
of  that  has  been  too  often  demonstrated.  Hence  he  has  selected 
for  the  experiment  a  group  of  rocks — pre-Devonian— which  no 
chemist  can  analyse  in  their  unmetamorphosed  condition. 
In  the  paper  now  before  the  Society  General  McMahon  had 
adduced  some  valuable  additional  evidence  in  favour  of  the  eruptive 
origin  of  the  Dartmoor  Granite,  and  he  had  strikingly  pointed  out 
the  almost  insuperable  difficulties  presented  by  the  unaltered  con¬ 
dition  of  the  Culm  Measures.  His  explanation  of  the  pseudo¬ 
bedding  structure  in  the  granite  was  well  worthy  of  attention. 
Mr.  W.  W.  Watts  observed  that  one  of  the  main  objects  of  Mr. 
Ussher’s  paper  "was  to  point  out  that  the  deflections  in  strike  of 
the  Culm  and  Devonian  rocks  would  be  explained,  if  there  were 
a  mass  of  rock  in  the  position  of  the  Dartmoor  Granite  in  post- 
Carboniferous  times ;  and  that  Mr.  Ussher  had  only  asserted  the  re¬ 
fusion  of  very  small  portions  of  the  edge  of  the  granite-mass  to  form 
the  elvan  and  granite-dykes. 
Mr.  Teall  wished  to  point  out  that  the  main  object  of  Mr.  Ussher’s 
paper  was  to  describe  the  Culm  Measures  and  their  stratigraphical 
relations  to  the  granite.  (The  President  here  reminded  Mr.  Teall 
that  the  part  of  Mr.  Ussher’s  paper  dealing  with  the  Culm  Measures 
was  more  or  less  of  the  nature  of  an  official  memoir  ;  whereas  the 
theory  criticized  was  the  main  point  in  that  portion  of  the  paper 
for  which  the  Author  was  alone  responsible.) 
1  [After  the  above  was  set  up  in  type,  a  friend  pointed  out  to  me  the  follow¬ 
ing  passage  in  Sterry  Hunt’s  ‘  Mineral  Physiology  and  Physiography,’  p.  273  : 
'In  this  connexion  I  venture  to  recall  the  attention  of  geologists  to  a  pheno¬ 
menon  already  described  both  by  Dr.  Shaler  and  myself,  apparently  due  to 
superficial  alternations  of  temperature  on  certain  crystalline  rocks,  which  have 
resulted  in  establishing  in  them,  to  a  considerable  depth,  a  series  of  rifts  or 
divisional  planes  parallel  to  the  present  surface,  which  are  well  known  to 
quarrymen.’ — July  11th,  1893.] 
2  d  2 
