150 
MISS  C.  A.  RAISIN'  OR  YARIOLITE  OF 
[May  1893,. 
a  zone  of  vesicles  radially  elongated,  as  if  gases  had  been  pre^  ented 
from  escaping.  Thus,  evidently,  a  crust  was  formed  at  an  early 
period,  before  the  structures  were  completely  solid.  Smaller  sphe¬ 
roids  also  are  enclosed  within  larger  (fig.  3,  b),  illustrating  successive 
Fig.  3. — Spheroids  from  the  cliff  south  of  Forth  Orion. 
b  =  zone  vith  reddish  sphe- 
rulites. 
c  =  interior,  of  purplish 
basalt. 
A  and  B  are,  roughly,  about  To 
natural  size,  but  the  spheroids 
vary  considerably  in  magnitude. 
stages  in  contraction,  as  was  demonstrated  by  Prof.  Bonney  :t 
while  in  a  green  diabase  (probably  an  ash)  north  of  Porth  Oer  the 
enclosing  shells  are  rhomboidal.1 2 3  The  outer  layers  of  a  spheroid 
are  generally  schistose  and  shining,  consisting  of  palagonite  or  of  a 
chloritic  or  serpentinous  aggregation,  which  doubtless  represents  an 
originally  glassy  exterior.  A  similar  matrix  surrounds  fragments 
in  certain  breccias.  The  foliated  structure  is  not  necessarily  due  to 
subsequent  pressure,  but  might  originate  by  slight  rolling  of  the 
spheroids,  if  the  crust  were  not  completely  solid."  The  still  plastic 
magma  would  be  squeezed  out  along  surfaces  of  weakness. 
1  Quart.  Journ.  Geol.  Soc.  vol.  xxxii.  (1876)  pp.  151,  153,  and  fig.  13. 
2  Compare  a  structure  in  shale  altered  by  contact,  described  by  Mr.  Harter, 
ibid.  vol.  xhv.  (.1888)  p.  450. 
3  J.  W.  Gregory,  ibid.  vol.  xlvii.  (1891)  p.  60. 
