148 
MISS  C.  A.  RAISIN  ON  VARIOLITE  OR 
[May  1893, 
ments  of  purple  basalt  have  been  enclosed  at  places  in  a  later  magma. 
In  other  examples  where  the  two  types  are  not  clearly  separated,  a 
tendency  in  this  direction  is  indicated  by  a  development  of  spheru- 
lites,  and  is  even  slightly  marked  in  fluxion-bands. 
Partial  analyses  of  two  of  this  rather  varied  group  of  rocks  have 
been  prepared  in  the  laboratory  at  University  College,  through  the 
kindness  of  Dr.  Plimpton.  The  first  apparently  belongs  to  the  class 
of  not  very  basic  basalts,  and  this  is  representative  of  my  4  ferru¬ 
ginous’  type  of  rock  (the  other  type  has  not  been  analysed).  The 
second  represents  an  abnormal  variety,  a  kind  of  palagonite. 
Si02 
A1203 
Pe203 
FeO  . 
CaO  . 
MgO  . 
co2  . 
Hygroscopic  water  ... 
Alkalies  and  water "[ 
by  difference  ...  J 
I. 
II. 
52-43 
29-78 
23-84 
17-72 
11-34 
o-oo 
2-68 
7*55 
7-88 
111 
12-77 
6*00 
6-80 
... 
0-35 
9-07 
10-68 
100-00  100-00 
I.  Yariolite  from  cliff  north  of  Porth  Oer.1 — This  rock  exhibits  a 
narrow  zone  of  distinct  spherulites,  with  gradations  to  a  mass  con¬ 
fusedly  crystallized,  as  shown  in  Pl.  I.  fig.  5.  The  specimen  analysed 
came  from  the  latter  part,  in  which  small  amygdules  occur  at 
intervals,  consisting  largely  of  a  carbonate.  This  deposit,  however, 
would  form  only  a  small  proportion  of  the  mass,  and  would  not  very 
materially  affect  the  results  of  the  analysis. 
II.  Green  palagonitic  rock  limiting  dull  red  basaltic  spheroids, 
from  the  boss  on  the  beach  north  of  Porth  Oer  (see  p.  151).2 
III.  Spheroidal  Structure. 
The  igneous  rocks  are  often  jointed  into  rhomboidal  columns 
weathering  to  roughened  ends  ;  and  spheroidal  structure,  which  was 
noted  by  Prof.  Bonney  at  Porth-din-lleyn,3  is  well  shown  at  many 
1  Separate  analyses  of  this  rock  were  obtained  by  Mr.  C.  J.  Nix  and  by 
Mr.  W.  Gatborne  Young,  the  two  results  being  to  a  great  extent  corroborative. 
I  am  further  indebted  to  Miss  E.  Aston,  B.Sc.,  for  kindly  making  an  inde¬ 
pendent  estimate  of  the  quantities  in  a  hydrochloric  acid  extract : — Fe.,03 
+A1203  3-73,  OaO  6-87,  MgO  1*06,  C02  6-00. 
2  For  the  analysis  of  this  rock  I  have  to  thank  Miss  J.  M.  Hayward,  B.Sc., 
who  also  made  a  separate  calculation  of  the  carbonates  present  in  the  rock. 
These  amounted  to  2088,  and,  on  the  hypothesis  that  they  were  all  infiltrations, 
the  percentages  in  the  original  rock  would  be : — Si02  38"22,  Al203-|-Fe203 
34'67,  MgO  13‘30,  with  alkalies  and  water  amounting  to  13"70.  As  the 
quantity  of  water  (not  determined  with  precision)  was  not  more  than  about  one 
third  of  this  last  amount,  the  high  percentage  of  alkali  with  a  low  percentage  of 
silica  suggests  the  enquiry  whether  nepheline  could  have  been  an  important 
constituent  in  the  original  rock. 
3  Quart.  Journ.  Geol.  Soc.  vol.  xxxvii.  (1881)  p.  50.  For  a  general  account 
of  such  structures,  see  ibid.  vol.  xxxii.  (1876)  pp.  140-154. 
