94 
PROF.  T.  G.  BONNET  ON  SOME  SCHISTOSE  [Feb.  1 893, 
5.  On  some  Schistose  ‘  Greenstones  ’  and  allied  Hornblendic 
Schists  from  the  Pennine  Alps,  as  illustrative  of  the  Effects  of 
Pressure-metamorphism.  By  Prof.  T.  G.  Bonnet,  D.Sc.,  LL.D., 
F.B.S.,  Y.P.G.S.  (Bead  December  7th,  1892.) 
That  a  rock  originally  angitic  may  become  bornblendic  is  well 
known,  but  the  cause  of  the  change  is  less  certain.  Some  authors 
seem  disposed  to  attribute  it,  almost  as  a  matter  of  course,  to  the 
direct  effects  of  pressure  ;  that  it  is  so  in  some  cases  can  hardly  be 
doubted,  but  that  this  explanation  universally  applies  mast  not  be 
hastily  assumed.  It  may,  for  instance,  be  demonstrated,  as  in  the 
classic  case  of  the  Scourie  Dyke,  that  a  dolerite  has  been  converted 
into  a  hornblende-schist,  but  the  question  of  the  efficient  cause  may 
still  remain  open.  At  the  Lizard,  as  the  present  author  has  en¬ 
deavoured  to  show,  augite  is  replaced  by  hornblende,  as  in  certain 
of  the  gabbros,  under  circumstances  which  make  it  highly  improbable 
that  pressure  has  been  operative  to  any  appreciable  extent.1  .  Hence, 
although  schistose  4  greenstones  ’  have  already  attracted  considerable 
attention,2  it  may  be  worth  while  to  describe  certain  cases  in  which 
pressure  must  have  been  an  important  agent  of  change,  in  the  hope 
of  ascertaining  some  mineral  or  structural  characteristics  which  may 
help  us  in  recognizing  metamorphism  due  mainly  to  mechanical 
causes.3 
The  first  group  of  specimens  to  be  noticed  in  the  present  paper 
was  obtained  in  the  summer  of  1891,  when  Mr.  J.  Eccles  and  myself 
spent  some  time  at  Saas  Fee,  in  the  A  ispthal.  The  structure  of  the 
great  mountain-mass  drained  by  the  two  branches  of  the  Yisp  is 
rather  complicated.  The  rocks  are  almost  wholly  crystalline: 
gneisses  appear  to  occupy  the  lowest  position,  and  to  be  succeeded, 
perhaps  discordantly,  by  a  mass  of  schists,  calcareous,  micaceous, 
and  quartzose  —  rocks  clearly  of  sedimentary  origin,  but  highly 
metamorphosed.  In  some  districts  green  schists,  apparently  inter- 
stratified,  form  part  of  the  latter  group  ;  in  others— as  in  .this — 
the  same  rock  occupies  a  considerable  area,  is  by  no  means  uniform 
in  character,  and  its  relations  to  the  other  rocks  are  more  uncertain. 
The  Swiss  geological  surveyors  evidently  found  it  a  difficulty.  On 
the  western  side  of  the  chain  of  the  Mischabelhorner  (i.  e.  in  the 
environs  of  Zermatt)  three  tints  are  employed,  indicating  respectively, 
1  Quart.  Journ.  Geol.  Soc.  vol.  xlvii.  (1891)  p.  489. 
2  See,  for  instance,  Teall,  ‘  Brit.  Petrogr./  pp.  161,  198,  242,  etc.,  and  Hyland, 
Geol.  Mag.  for  1890,  p.  205.  These  authors,  and  others  to  whom  they  refer, 
have  already  argued  for  certain  of  the  conclusions  adopted  above,  but  seeing 
that  in  some  cases  the  phenomena  are  not  placed  in  relation  as  cause  and  effect,, 
and  that  in  others  pressure  seems  to  be  too  readily  assumed  as  the  agent  of 
change,  I  have  discussed  the  question  at  some  length. 
3  Dr.  Callaway,  Quart.  Journ.  Geol.  Soc.  vol.  xlv.  (1889)  pp.  480  et  seq., 
claims  biotite  as  a  result  of  £  shearing  ’  in  a  diorite,  but  I  am  unable  to  accept 
some  important  details  in  the  history  of  its  development  as  given  by  him. 
