464 
bey.  j.  e.  blaxe  on  eelsites  and 
[Aug.  1893*, 
side  of  its  longer  axis  ;  its  relations  to  the  slate  are  not  further 
indicated ;  hut,  seeing  that  it  lies  in  the  continuation  of  the  line 
already  shown  to  be  a  fault,  its  appearance  may  be  very  well  accounted 
for  by  some  complication  and  breaking  up  of  this,  without  assuming 
that  the  felsite  is  intrusive. 
X.  Disteibution  oe  the  Pelsites  in  the  Cambeian  Seeies. 
It  may  be  useful  to  summarize  our  present  knowledge  on  this  sub¬ 
ject,  so  as  to  be  able  to  appreciate  how  abundant  were  acid  volcanic 
eruptions  in  the  Cambrian  period.  Probably  the  oldest  known  acid 
eruption  is  that  in  Baron  Hill  Park,  near  Beaumaris,  which  underlies 
everything  Cambrian  and  is  in  contact  with  the  Monian  rocks. 
Next  in  order  comes  the  mass  between  Bangor  and  Caernarvon,  which 
lies  below  every  rock  known  on  the  mainland.  Then  there  is  a 
small  mass  on  the  eastern  slopes  of  Dinas  Dinorwig  which  cannot 
be  far  from  the  base  ;  it  has  its  own  appropriate  conglomerate,  but 
may  correspond  to  the  Tairffynnon  Breccia.  To  about  this  period 
may  belong  the  igneous  rocks  of  Craig-y-Dinas  and  of  Gwyrfai 
River,  and  the  two  isolated  bosses  lying  near  Iihos  Tryfaen : 
certainly  this  is  the  age  of  some  of  the  felsite  on  the  west  of  Moel 
Tryfaen ;  and,  owing  to  faulted  conditions,  this  or  any  later  age 
may  be  assigned  to  the  large  masses  of  Mynydd-y-Cilgwyn  and 
Moel  Smythaw.  It  seems  difficult  to  believe  that  the  two  masses  on 
either  side  of  Llyn  Padarn,  i.  e.  Clegyr  and  Careg  Lefain,  are  of 
different  ages ;  but  the  former  is  followed  by  some  of  the  Banded  Slates, 
and  the  latter  has  Purple  Slates  on  both  sides ;  it  may  be  noted  also 
that  on  Clegyr  dykes  of  a  newer  felsite  are  seen  traversing  the 
more  decayed  mass.  It  is  to  the  Clegyr  mass  that  we  must  assign 
the  formation  of  the  Bangor  Conglomerates,  and  that  some  part  of 
Moel  Smythaw  must  also  belong  here  is  suggested  by  the  felsite 
No.  6,  which  occurs  in  the  Moel  Tryfaen  adit  in  the  midst  of  the 
Banded  Slates.  Of  the  same  age  as  Careg  Lefain  is  the  mass 
of  Moel  Goronwy,  and  either  to  this  or  a  later  eruption  must  be 
assigned  the  felsite  in  the  Penrhyn  Tunnel. 
This  is  a  tolerably  persistent  group  of  eruptions,  if  we  are 
justified,  as  I  think  we  are,  in  assigning  them  to  the  periods  which 
their  stratigraphical  position  indicates.  More  than  this,  the  coarse 
Bangor  Breccias  and  the  brilliant  grains  of  quartz  in  many  of  the 
Laminated  Grits  indicate  their  derivation  either  from  the  felsites  or 
from  rocks  broken  up  by  them  ;  and  even  the  Banded  Slates  are  just 
such  rocks  as  might  be  derived  from  the  finer  dust,  while  the 
Purple  Slates  themselves  are  peculiar  rocks  which  may  well  be 
obtained  from  volcanic  material.  Of  course  all  slates  are  originally 
derived  from  either  plutonic  or  volcanic  masses,  but  those  of  the 
Cambrian  period  seem  to  be  more  directly  so  derived. 
