200 
MESSES.  H.  FOX  AND  J.  J.  H.  TEALL  ON  [May  1 893, 
of,  conformable  bedded  rocks,  some  beds  have  been  converted  into 
that  peculiar  variety  of  hornblende-schist  which  characterizes  the 
locality  [Porthalla],  others  have  been  converted  into  true  serpentine, 
and  others  again  into  a  rock  of  intermediate  character.” 1  Thus  the 
two  opposing  views  which  had  been  indicated  by  De  la  Beche  were 
supported,  with  some  modifications,  as  independent  hypotheses  by 
Prof.  Bonney  and  Mr.  Collins. 
During  portions  of  the  summers  of  1890  and  1891  we  devoted 
some  time  to  the  examination  of  the  coast  near  Ogo  Dour — a  locality 
which  seemed  likely  to  furnish  important  evidence  on  the  vexed 
question  of  the  relations  of  hornblende-schist  to  serpentine.  As 
the  results  at  which  we  have  arrived  do  not  precisely  agree  with  those 
of  previous  observers,  and  yet  at  the  same  time  tend  to  bring  the 
conflicting  views  into  more  or  less  harmony,  we  venture  to  hope 
that  a  description  of  the  facts  which  have  influenced  us  will  be  of 
interest  to  this  Society. 
Briefly  stated,  our  conclusions  are  as  follows : — We  agree  with 
Prof.  Bonney  that  the  serpentines  have  in  all  cases  been  produced 
by  the  alteration  of  rocks  in  which  olivine  was  an  important  con¬ 
stituent,  but  we  believe  that  these  rocks  formed  part  and  parcel  of 
the  foliated  series  to  which  the  hornblende-schists  belong,  and  that 
the  apparent  evidences  of  intrusion  of  serpentine  into  schist,  in  the 
district  in  question,  are  consequences  of  the  folding  and  faulting  to 
which  the  rocks  have  been  subjected  since  the  banding  was  produced. 
Whether  these  conclusions  will  be  found  to  apply  to  all  portions  of 
the  Lizard  district  or  not,  is  a  question  on  which  we  desire  at  present 
to  offer  no  definite  opinion. 
In  treating  the  subject  we  will  in  the  first  place  deal  with  the 
original  relations  of  the  serpentine  and  hornblende-schists,  and  then 
with  the  effects  of  folding  and  faulting. 
(a)  Interbanding  of  Schist  and  Serpentine. 
Absolutely  undisturbed  junctions  of  hornblende-schist  and  serpen¬ 
tine  are  extremely  rare  in  the  Lizard  District,  especially  when  both 
rocks  present  their  typical  characters,  and  are  developed  on  a  large 
scale.  In  the  district  in  question  the  definite  interbanding  of  these 
rocks  may,  however,  be  observed  in  more  than  one  locality,  and  hand- 
specimens  may  be  obtained  in  which  both  rocks  are  represented. 
As  an  illustration  we  may  refer  to  the  promontory  at  Potstone 
Point  (see  Map,  p.  203).  Serpentine  is  here  the  prevailing  rock,  but  it 
contains  thick  and  thin  bands  of  schist.  The  interbanding  or  inter¬ 
lamination,  as  it  may  be  termed,  is  often  on  so  minute  a  scale  that 
microscopic  sections  may  be  prepared  to  illustrate  it.  One  hand- 
specimen,  from  which  a  section  has  been  cut,  shows  a  conspicuous 
band  of  hornblende-schist,  about  \  inch  in  thickness,  traversing  the 
specimen  throughout,  and  other  much  thinner  and  less  persistent 
folia.  The  main  mass  of  the  specimen  is  composed  of  olivine,  horn¬ 
blende,  serpentine  after  one  or  both  of  these  minerals,  the  charac- 
1  Geol.  Mag.  for  1885,  p.  299. 
