192  Geological  Society. 
an  extension  of  the  present  course  of  the  Iiiver  Tagus,  was  exca- 
vated. The  margin  of  the  lake  was  probably  formed  by  the  granite 
of  Das  Vargans  and  Cunheira.  There  is  evidence  that  the  general 
level  of  the  lake-bed  was  nearly  that  of  the  outer  sea,  and  that  the 
sea-waters  gained  occasional  access  to  the  lake  during  the  earlier 
stage  of  its  formation.  The  lake  was  eventually  drained  by  the 
channel  cut  by  the  Tagus  at  the  harbour  of  Lisbon,  upon  the 
elevation  of  the  land  to  about  its  present  level. 
2.  *  The  Geological  Structure  of  the  Sgurr  of  Eigg.'"  By  Alfred 
Harker,  M.A.,  F.Ii.S.,  F.G.S. 
The  pitchstone  which  forms  the  Sgurr  of  Eigg  is  a  massive  sheet, 
some  400  feet  thick,  reposing  with  discordance  upon  the  succession 
of  alternating  basalts  and  dolerites  which  make  up  the  greater  part 
of  the  island.  The  lower  surface  of  the  pitchstone  is  irregularly 
undulating,  and  in  two  places  fragmental  accumulations  are  seen 
immediately  beneath  it.  The  generally-received  interpretation 
regards  the  pitchstone  as  a  lava-flow,  or  series  of  flows,  occupying 
an  old  river-valley  excavated  in  the  basalts,  and  the  fragmental 
deposits  have  been  regarded  as  river-gravels  of  the  pitchstone-age. 
This  is  the  view  put  forward  by  Sir  Archibald  Geikie. 
Alter  a  detailed  survey  of  the  ground,  the  author  finds  it 
impossible  to  accept  this  view,  and  he  gives  reasons  for  considering 
the  pitchstone  to  be  intrusive.  The  form  of  its  base,  as  mapped 
out,  does  not  seem  to  be  reconcilable  Avith  that  of  a  river-valley, 
and  its  character  is  that  of  an  intrusive  junction  rather  than  an 
erosion -surface.  The  fragmental  deposits  are  believed  to  be  of 
volcanic  origin  and  of  the  basalt-age.  The  one  exposed  at  the 
seaward  termination  of  the  ridge  is  a  volcanic  agglomerate,  probably 
filling  a  small  vent.  The  other,  seen  at  the  southerly  base  of  the 
Sgurr,  is  a  bedded  agglomerate,  partly  rearranged  by  water-action. 
The  Torridonian  and  Oolitic  sandstone-blocks  which  are  abundant 
in  it  are  held  to  have  been  brought  up  from  below,  and  fossil  wood 
of  Oolitic  age  has  been  brought  up  in  the  same  manner.  The 
absence  of  fragments  of  the  sill-dolerites  (themselves  younger  than 
the  lavas,  but  cut  off  by  the  pitchstone)  in  both  accumulations 
seems  to  assign  them  unequivocally  to  the  age  of  the  basalt,  and 
their  conjunction  with  the  pitchstone  must  then  be  considered 
accidental. 
The  conclusions  arrived  at  bring  the  rock  of  the  Sgurr  of  Eigg 
into  relation  with  the  other  British  Tertiary  pitchstones,  which  are 
all  intrusive.  Thus  also  is  avoided  the  difficulty  of  assuming  a 
great  erosion  in  inter-volcanic  times,  a  hypothesis  for  which  the 
supposed  river-valley  was  the  sole  evidence. 
