()16  Geological  Society. 
quartz-dolerifces  and  granophyres,  for  which  no  satisfactory  name 
seems  to  exist.  There  is  also  a  development  of  peculiar  rock-types 
as  the  result  of  the  re-mixing  of  previously-differentiated  partial 
magmas  of  an  acid  and  a  basic  character  respectively.  A  study  of 
the  distribution  of  different  types  of  granophyric  structure  shows  a 
certain  regularity  of  arrangement,  and  an  attempt  is  made  to 
reconcile  these  with  known  physical  laws,  especially  with  reference 
to  eutectics  ;  and  it  is  concluded  that  the  structure  is  the  result  of 
crystallization  under  conditions  intermediate  between  those  which 
produce  typical  plutonic  and  hypabyssal  rocks. 
February  7th.— J.  E.  Marr,  Sc.D.,  F.R.S.,  President, 
in  the  Chair. 
The  following  communications  were  read  : — 
1.  'The  Carboniferous  Limestone  (Avonian)  of  the  Mendip  Area 
(Somerset),  with  especial  reference  to  the  Palseontological  Sequence.' 
By  Thomas  Franklin  Sibly,  B.Sc,  F.G.S. 
2.  '  The  Igneous  Rocks  of  the  Eastern  Mendips.'  Bv  Prof. 
Sidney  Hugh  Reynolds,  M.A.,  F.G.S. 
The  igneous  rocks  associated  with  the  Old  Red  Sandstone  of  the 
Men  dips  are  exposed  along  the  crest  of  the  range  from  Beacon  Hill 
on  the  west  to  near  Downhead  on  the  east,  a  distance  of  rather 
more  than  two  miles.  Hitherto  they  have  always  been  regarded 
as  intrusive,  but  the  opening  of  some  new  excavations  has  shown 
that  they  are  associated  with  a  considerable  thickness  of  tuffs,  and 
are  in  all  probability  contemporaneous  lava-flows. 
The  exposures  show  a  division  into  three  sections — those  of 
Beacon  Hill,  Moon's  Hill,  and  Downhead ;  and  a  large  quarry  has 
been  opened  in  the  trap  in  each  section.  The  trap,  which  can  be 
traced  fairly  continuously  from  one  end  of  the  area  to  the  other,  is 
very  uniform  in  character,  consisting  (as  already  noted  by  Dr.  Teall) 
of  a  non-amygdaloidal  pyroxene-andesite,  which  usually  contains 
augite  in  addition  to  enstatite.  A  fine  section  of  tuff  some  100  feet 
thick  is  seen  lying  with  perfect  conformity  below  the  trap  in  the 
Xew  Quarry  near  Stoke  Lane ;  and  an  interesting  little  exposure  of 
tuff,  remarkable  for  the  numerous  rounded  blocks  of  trap  present, 
is  seen  in  the  excavation  for  the  rifle-butts  on  Beacon  Hill.  The 
tuff  here  dips  under  the  Old  Red  Sandstone  to  the  north.  Although 
the  tuff  is  seen  in  situ  only  at  the  above  two  points,  loose  pieces 
have  been  met  with  at  a  number  of  other  spots  all  along  the 
southern  outcrop  of  the  trap,  and  point  clearly  to  the  occurrence 
of  a  continuous  band  underlying  it. 
Though  no  sedimentary  rocks  are  seen  in  direct  contact  with 
those  of  the  igneous  series,  outcrops  of  Old  Red  Sandstone  com- 
pletely surround  the  exposures  of  trap  and  tuff,  and  occur  in  such 
close  relation  to  them  as  to  leave  little  room  for  doubt  that  the 
igneous  series  is  of  Old-Red-Sandstone  age.  On  the  other  hand, 
Silurian  fossils  were  met  with  below  the  igneous  series  at  a  point 
to  the  west  of  Downhead,  and  render  it  possible  that  the  igneous 
rocks  may  be  of  Silurian  age,  and  the  equivalents  of  those  which 
are  exposed  at  Tortworth. 
