Vol.  49.] 
OF  MUSCOVITE-BIOTITE  GNEISS. 
353 
uniform  horizon.  This  permeating  sill  is  so  inextricably  interwoven 
with  the  cyanite-schists  and  other  rocks  with  which  it  is  associated 
that  it  was  not  until  Mr.  Teall  showed  how  the  minute  structures 
of  the  thin  intrusions  were  largely  those  of  original  igneous  rocks 
that  it  became  possible  to  believe  that  the  whole  complex  mass  was 
not  of  one  age  and  of  one  mode  of  origin.  If  this  mode  of  intrusion 
as  sills  or  laccolites  be  admitted,  it  follows  that  one  of  the  chief 
factors  in  increasing  or  decreasing  metamorphism  of  the  rocks 
affected  must  be  the  variation  in  depth  of  the  sills  below  the  surface. 
In  the  case  of  ordinary  granite-intrusions  it  is  customary  to  measure 
the  metamorphism  by  the  distance  from  the  edge  of  the  granite  at 
the  surface ;  but  in  the  special  case  under  consideration  it  must 
rather  be  measured  by  the  height  above  the  top  of  the  intrusion. 
The  intensity  of  the  metamorphism  is  doubtless  largely  due  to  the 
great  depth  below  the  surface  of  the  rocks  affected  by  the  intrusion. 
The  importance  of  this  factor  is  eonclusivety  shown  by  Dr.  Barrois 
in  his  papers  on  the  granites  of  Britanny.  The  same  mass  is  there 
proved  to  have  affected  Cambrian,  Silurian,  and  Devonian  rocks  ; 
but  the  metamorphism  in  the  Cambrian  strata  is  more  pronounced 
than  in  the  Silurian  aud  still  more  than  in  the  Devonian.  In  other 
words,  the  lowest  rocks  are  most  affected. 
In  estimating  the  difference  in  depth  at  which  the  change  took 
place  in  the  rocks  of  the  different  periods,  account  must  be  taken 
not  only  of  the  thickness  of  the  deposits,  but  also  of  the  folding 
to  which  they  had  been  subjected  before  the  intrusion  of  the  granite. 
If  the  folds  were  of  great  depth,  the  older  rocks  would  be  buried 
under  a  cover  of  newer  strata,  the  original  thickness  of  which 
would  be  enormously  increased.  Now,  in  the  area  to  which  this 
paper  refers,  it  can  be  shown  that  the  rocks  affected  are  the  -lowest 
in  the  series  and  that  they  have  been  enormously  folded,  the  depth 
of  the  major  folds  being  apparently  never  less  than  2000  feet. 
Thus  the  occurrence  of  sillimanite  and  coarsely  crystalline  gneisses 
over  large  areas  may  be  easily  explained  by  the  great  depth  at  which 
the  metamorphism  took  place. 
Bocks  of  the  kind  described  in  this  communication  are  usually 
of  great  antiquity,  and  their  especial  features  are  regarded  by 
some  geologists  as  evidence  of  the  existence  in  early  geological 
times  of  physical  conditions  distinctly  different  from  those  which 
now  prevail.  The  considerations  which  have  been  advanced  above 
suggest  that  these  special  features  may  after  all  be  due  to  the  depth 
in  the  earth’s  crust  at  which  the  metamorphism  took  place,  rather 
than  to  any  physical  conditions  peculiar  to  early  geological  time. 
A  comparison  of  the  rocks  of  Shap  and  New  Galloway  with 
those  of  the  district  under  consideration  shows  that  the  difference 
between  them  is  one  of  degree ,  not  one  of  kind,  and  strengthens 
Dr.  Barrois’s  conclusion  that  “regional  metamorphism  and  contact- 
metamorphism  are  much  the  same  thing.”  The  great  geological 
age  of  most,  if  not  of  all,  of  the  areas  where  phenomena  similar  to 
those  we  have  referred  to  may  be  observed  receives  a  simple  explana¬ 
tion  in  the  fact  that  an  enormous  amount  of  time  is  required  to 
remove  the  overlying  material  by  denudation. 
