408  DR.  C.  CALLAWAY  ON"  THE  ORIGIN  OF  THE  [Aug.  1893, 
transparent  mineral  are  also  present.  These  granules  and  micro- 
liths  may  perhaps  belong  to  the  chlorite  family.  Many  of  them 
run  in  streams  lying  in  the  direction  of  the  general  schistosity. 
Associated  with  the  chlorite  is  a  small  proportion  of  white  mica. 
It  is  either  in  microliths  with  lath-shaped  sections,  or  in  larger 
crystals  more  or  less  rounded,  but  sometimes  with  distinct  basal 
cleavage  and  laminae  projecting  in  a  step-like  manner.  Elongation 
is  sometimes  parallel,  sometimes  perpendicular,  to  P.  In  both 
cases,  the  longer  axes  lie  in  the  planes  of  schistosity,  and  accentuate 
the  foliated  structure. 
The  ‘quartz-felspar’  is  very  clear,  and  much  of  it  has  assumed  a 
true  mosaic  structure.  The  granules  of  the  mosaic,  which  are  often 
very  minute,  have  usually  sinuate,  occasionally  foliate  contours. 
The  fragmental  origin  of  some  of  the  quartz-felspar  is  still  evident; 
but  on  the  whole  there  is  a  less  clastic  look  about  the  slide,  the 
angles  of  the  fragments  being  often  rounded  off.  Where  they  have 
been  in  contact,  they  have  coalesced,  and  a  broken  stream  of  the 
minute  microliths  and  granules  of  chlorite  (?)  marks  the  boundary 
between  them,  the  quartz-felspar  polarizing  differently  at  the  two 
sides  of  the  line.  Sometimes  the  minute  chlorite  (?)-forms  are 
enclosed  in  simple  granules  of  quartz-felspar,  where  a  union  of 
fragments  is  unlikely.  It  seems  therefore  evident  that  the  quartz- 
felspar  has  been  more  or  less  fused  or  dissolved,  so  as  to  permit  not 
only  of  the  coalescence  of  fragments,  but  of  the  inclusion  of  foreign 
minerals. 
The  last  rock  passes  gradcitim  within  2  feet  into  a  well-marked 
schist,  showing  in  hand-specimens  a  clear  foliation  with  glistening 
mica-surfaces.  The  next  slide  illustrates  the  minute  structure. 
Ho.  486  (fig.  3). — The  slide  is  quite  free  from  cloudiness,  and 
the  parallelism  of  the  constituents  is  very  marked.  This  is  seen  in 
several  lines  of  minute  opaque  granules,  which  possibly  indicate  the 
position  of  shear-nlanes,  as  well  as  in  the  arrangement  of  the  mica, 
chlorite,  and  quartz-felspar.  The  chlorite  and  the  white  mica  are 
present  in  about  equal  proportions.  They  sometimes  pass  into 
each  other  by  imperceptible  gradations,  the  green  colour  of  the 
chlorite  shading  into  the  transparency  of  the  mica  in  ordinary 
light,  as  the  dark  shades  of  the  interference-colours  of  the  former 
graduate  under  crossed  nicols  into  the  bright  hues  of  the  latter. 
As  I  have  already  discussed  the  passage  of  chlorite  into  white  mica 
(Ho.  II. ,  p.  488),  I  need  not  here  repeat  the  details  of  this  important 
mineral  change  ;  but,  as  tending  to  confirm  the  evidence  offered  in 
these  papers,  I  may  quote  Prof.  Judd’s1  observation  of  a  gradual 
transition  from  basic  lime-felspar  towards  an  acid  alkali-felspar, 
the  optical  characters  changing  pari  passu  with  the  diminution  of 
lime. 
That  the  white  mica  was  either  formed  from  the  chlorite  or 
generated  contemporaneously,  is  evident  from  the  fact  that,  in  this 
slide  (as  in  many  others),  the  two  minerals  are  frequently  associated 
1  Quart.  Journ.  Geol  Soc.  vol.  xlv.  (1889)  p.  180. 
