Glassy  and  felsitic  eruptives,  particularly  of  the  liparite  and
quartz  porphyry  groups,  frequently  show  spherulitic  masses  of
all  sizes,  from  microscopic  to  several  inches  or  even  feet  in
diameter,  usually  with  a  well-defined  radiating  structure  and
which  are  due  to  incipient  crystallization.  Such  are  known  as
spherulites,  and  hence  rocks  in  which  they  occur  are  described
as  spherulitic.1

A  concretionary  structure  is  not  infrequently  developed  in
rocks  either  as  a  primary  structure  or  as  due  to  segregating
processes  acting  subsequent  to  the  formation  of  the  rocks  in

1  The  structure  and  origin  of  these  forms  has  been  worked  out  in  detail  by
Whitman  Cross.  Bull.  Philosophical  Society  of  Washington,  Vol.  XI,  1891,
pp.  411-462.
36          PHYSICAL   AND   CHEMICAL  PROPERTIES   OF  ROCKS

which  they  are  found.  Many  of  the  forms  thus  developed  are
peculiarly  deceptive,  and  it  may  not  be  out  of  place  to  enter
into  a  discussion  of  their  nature  and  origin  with  some  detail.
