Vol.  49.] 
OF  MTJSCOVITE-BIOTITE  GNEISS. 
357 
rock  differing  in  composition  from  that  in  which  the  sillimanite  and 
cyanite  were  developed. 
General  McMahon  congratulated  the  Author  on  the  interest  and 
importance  of  his  results.  Of  late  years  the  geological  mind  had 
been  so  fascinated  with  the  doctrine  of  dynamo-metamorphism  that 
thermo-metamorphism.  and  the  foliation  of  igneous  rocks  prior  to 
their  complete  consolidation  had  not  received  adequate  considera¬ 
tion.  It  was  encouraging  to  find  that  the  tide  had  now  turned. 
In  the  Himalayas,  where  the  eruptive  gneissose  granite  was  of  a 
strongly  porphvritic  type,  he  had  observed  in  some  places  that  the 
porphyritic  structure  was  well  maintained  ia  veins  sent  off  from  the 
main  mass  into  the  adjoining  sedimentary  rocks.  When  an  erup¬ 
tive  rock  had  been  moved  into  position  after  the  partial  crystalliza¬ 
tion  of  the  magma,  such  a  result  might  be  expected  ;  and  when  a 
mass  full  of  crystals  was  forced  through  the  jaws  of  a  fissure, 
parallelism  of  structure  and  the  crushing  and  crumpling  of  crystals 
would  inevitably  result. 
General  McMahon  also  alluded  to  observations  at  the  Lizard  made 
by  Prof.  Bonney  and  himself,  where  the  very  coarse-grained  gabbro 
was  not  in  the  main  mass  but  in  small  dykes — a  state  of  things 
analogous  to  the  Author’s  observations  regarding  his  ‘  giant  granite  ’ ; 
and  it  pointed  to  a  similar  cause,  namely,  to  the  moving  up  at  a 
later  stage  in  an  eruption  of  deeper-seated  portions  of  a  magma, 
the  crystallization  of  which  had  considerably  advanced. 
l)r.  Hicks  said  it  was  clear  the  results  referred  to  by  the  Author 
could  not  have  originated  from  one  cause,  but  during  a  period 
when  a  combination  of  circumstances  tended  to  favour  such  changes. 
Mechanical  movements  were  necessary,  if  only  to  produce  the  frac¬ 
tures  through  which  the  magma  and  the  heated  waters  and  vapours 
permeated  the  rocks.  The  recent  intrusions  could  have  had  little  or 
no  influence  in  producing  the  main  results,  as  the  evidence  clearly 
pointed  to  their  having  taken  place  at  a  much  earlier  period. 
Similar  pegmatite-veins  were  found  traversing  not  only  the  rocks  in 
the  Central  Highlands,  but  also  those  of  the  Horth-  western  Highlands, 
and  fragments  which  must  have  been  derived  from  such  veins 
occurred  frequently  in  the  Torridon  Sandstone.  He  was  satisfied  that 
the  metamorphism  of  these  rocks  dated  back  to  pre-Cambrian  time  ; 
but  there  was  no  reason,  of  course,  why  such  changes  should  not  have 
taken  place  since  that  time.  What  he  maintained  was  that  for 
hundreds  of  square  miles  rocks  of  similar  composition,  and  showing 
a  similar  state  of  change,  were  to  be  found,  and  that,  under  the 
circumstances,  it  was  legitimate  to  attempt  here  a  correlation  by 
petrological  characters.  The  differences  observed  were  in  the  main 
due  to  the  original  character  of  the  sediments,  and  to  the  readiness 
of  the  materials  to  undergo  change.  He  congratulated  the  Author 
on  the  broad  and  liberal  manner  in  which  he  had  dealt  with  the 
several  questions  involved. 
Mr.  Mark  said  that,  in  their  joint  paper  on  the  Shap  Granite,  he 
and  Mr.  Harker  had  expressed  the  hope  that  their  work  would  be 
found  to  have  a  bearing  on  the  general  question  of  metamorphism. 
