﻿ON 
  SOME 
  NODULAR 
  FELSTONES 
  OF 
  THE 
  LLEYN. 
  247 
  

  

  15. 
  On 
  some 
  Nodular 
  Felstones 
  of 
  the 
  Lleyn. 
  

   By 
  Catherine 
  A. 
  Raisin, 
  B.Sc. 
  (Read 
  February 
  20, 
  1889.) 
  

  

  (Communicated 
  by 
  Prof. 
  T. 
  G. 
  Bonney, 
  D.Sc, 
  LL.D., 
  F.K.S., 
  F.G.S.) 
  

  

  I. 
  General 
  Succession 
  and 
  Age 
  op 
  the 
  Rocks. 
  

  

  (a) 
  Pen-y-chain. 
  

  

  (1) 
  Sections 
  near 
  the 
  South 
  Beach. 
  

  

  (2) 
  G-eneral 
  summary 
  of 
  overlying 
  rocks. 
  

  

  (3) 
  Series 
  near 
  Llym-gwyn. 
  

  

  (b) 
  Careg-y-defaid. 
  

  

  II. 
  Some 
  Petrological 
  Characters 
  op 
  the 
  Eocks. 
  

  

  (1) 
  Mineralogical. 
  

  

  (2) 
  Perlitic, 
  Spherulitic, 
  and 
  Micrographic 
  structures. 
  

  

  (3) 
  Silicification. 
  

  

  III. 
  Nodular 
  Structures. 
  

  

  (a) 
  Perlitic 
  Spheroids. 
  

  

  (b) 
  Concentric 
  structures 
  in 
  Nodules. 
  

  

  (c) 
  Agate 
  Nodules 
  ; 
  general 
  considerations. 
  

  

  (d) 
  Examples 
  of 
  Agate 
  Nodules. 
  

  

  (1) 
  Lithophysse 
  with 
  superimposed 
  chambers. 
  

  

  (2) 
  Nodules 
  with 
  external 
  ridges. 
  

  

  (3) 
  Nodules 
  developed 
  in 
  certain 
  Strata. 
  

  

  (4) 
  Occurrence 
  of 
  Amygdaloids. 
  

  

  (5) 
  Spheroidal 
  Crack 
  around 
  Nucleus. 
  

  

  Summary. 
  

  

  In 
  the 
  descriptions 
  which 
  have 
  been 
  given 
  of 
  various 
  igneous 
  

   rocks 
  from 
  the 
  Lleyn, 
  no 
  details 
  have, 
  I 
  believe, 
  been 
  published 
  of 
  

   the 
  two 
  small 
  outcrops 
  near 
  Pwllheli, 
  to 
  which 
  this 
  paper 
  refers. 
  

   Some 
  specimens, 
  which 
  I 
  collected 
  in 
  1885, 
  1 
  showed 
  to 
  Professor 
  

   Bonney, 
  and, 
  at 
  his 
  suggestion, 
  I 
  tried 
  to 
  make 
  a 
  more 
  thorough 
  

   examination 
  of 
  the 
  rocks. 
  The 
  two 
  masses 
  rise, 
  like 
  small 
  islands, 
  

   from 
  the 
  drift-covered 
  plain 
  around, 
  and 
  project 
  seawards 
  in 
  the 
  

   headlands 
  of 
  Pen-y-chain 
  and 
  Careg-y-defaid. 
  They 
  are 
  marked 
  

   on 
  the 
  Survey 
  map 
  as 
  consisting 
  of 
  intrusive 
  felspar 
  porphyry 
  with 
  

   agate 
  nodules. 
  I 
  propose, 
  first, 
  to 
  describe 
  the 
  general 
  types 
  of 
  

   rock, 
  and, 
  secondly, 
  to 
  notice 
  certain 
  structures, 
  especially 
  those 
  

   connected 
  with 
  the 
  nodules. 
  

  

  I. 
  General 
  Succession 
  and 
  Age 
  of 
  the 
  Bocks. 
  

  

  At 
  Pen-y-chain 
  the 
  rocks 
  show 
  distinct 
  evidence 
  of 
  stratification, 
  

   and 
  the 
  beds 
  seem, 
  on 
  the 
  whole, 
  to 
  dip 
  about 
  N.N.E., 
  although 
  

   with 
  irregularities, 
  such 
  as 
  might 
  be 
  expected 
  in 
  volcanic 
  masses. 
  

   Thus 
  the 
  lowest 
  strata 
  which 
  I 
  reached 
  were 
  the 
  rocks 
  near 
  the 
  

   south 
  or 
  south-westerly 
  point. 
  They 
  form 
  the 
  seaward 
  part 
  of 
  the 
  

   cliff, 
  against 
  which 
  the 
  long 
  beach 
  from 
  Pwllheli 
  terminates. 
  The 
  

   rock 
  is 
  darkly 
  veined, 
  with 
  subangular 
  fragments 
  of 
  whitish 
  felstone, 
  

  

  