Vol.  49.]  tue  lleyx,  axd  associated  yolcaxic  bocks.  149 
places,  the  spheroids  being  sometimes  as  mnch  as  6  feet  in  length, 
and  often  cushioned  in  appearance.1  Their  exterior  is  distinguished 
generally  by  characteristic  fracture  and  colour.2  It  is  fissured  in 
concentric  and  radial  directions,  the  interior  being  traversed  by  an 
irregular  rhomboidal  jointing  (fig.  1).  TIany  spheroids  also  contain 
Fig*  1* — Spheroid  of  basalt  f  rom  the  beach  north  of  Porth  Oer , 
showing  the  general  direction  of  the  cracks. 
a  b 
About  4  natural  size. 
a  =  outer  zone,  with  radial  fissures ;  b  =  interior,  with 
irregular  rhomboidal  jointings. 
Tig.  2. — Green  spheroidal  diabase,  as  exposed  on  the  surface  of 
a  rock  in  the  farmyard  at  Dwyrhos. 
About  natural  size. 
a  =  green  schistose  rock  ;  b  =  spherulitic  zone  near  the 
exterior  of  the  spheroid. 
1  Cole  and  Gregory,  op.  ctt.  vol.  xlyi.  (1890)  pp.  311,  312,  and  fig.  4. 
2  Variolitic  structure  is  often  connected  with  this  part  of  the  spheroid;  see 
below,  p.  155. 
