Vol.  49.]  CRYSTALLINE  SCHISTS  OF  THE  MALVERN  HILLS.  417 
occasional  replacement  of  felspar  by  zoisite  or  epidote,  and  the 
frequent  traces  of  kersantite-structure. 
The  traces  of  original  structure  in  a  rock  of  secondary  origin 
may  perhaps  be  appropriately  described  as  4  residua/  The  4  eyes  ’ 
of  an  augen- gneiss  or  augen-g abbro,  and  the  lenticular  cores  (den¬ 
ticles)  of'  granite  in  a  muscovite-gneiss  are  the  direct  result  of 
shearing.  But  the  residua  of  kersantite  in  a  biotite-gneiss  are 
patches  of  rock  which  have  escaped  advanced  molecular  changes, 
and  they  are  not  as  a  rule  sharply  distinguished  at  their  margins 
from  the  surrounding  mass. 
YI.  The  Mature  of  the  Shearing. 
To  remove  some  doubt  that  appears  to  exist  with  reference  to  my 
use  of  the  terms  4  shearing 9  and  4  shear-plane,’  I  may  state  that 
I  accept  the  definitions  of  those  writers  who  in  this  country  have 
been  among  the  pioneers  in  these  new  studies. 
Prof.  Lapworth,  describing 1  the  process  of  schist-making  in  the 
Highlands,  states  that  many  of  the  rocks  44  have  been  crushed, 
mashed,  and  dragged  out,  their  component  patches,  fragments, 
particles,  and  crystals  shearing  (or  moving  over  each  other  with  a 
differential  motion,  each  higher  layer  moving  a  little  farther  than 
the  one  below)  as  the  mass  gave  way.”  The  same  writer  defines 
44  shear-planes  ”  as  44  definite  planes  ”  along  which  44  the  rock  yielded 
to  the  excessive  pressure  and  torsion”  (op.  cit.  p.  114). 
The  Geological  Survey,  in  their  elaborate  memoir  on  the  Northern 
Highlands,2  adopt  a  similar  terminology.  The  lines  of  movement 
in  the  rocks  are  described  as  44  thrust-planes,  crush-lines,  or  lines 
of  shearing,  resulting  in  a  newer  foliation  ”  (p.  391).  In  certain 
of  the  schists,  each  44  shear-plane  ”  is  referred  to  (p.  435)  as  a  44  divi¬ 
sional  plane  or  foliation-surface,”  and  these  planes  are  described  a 
few  lines  before  as  the  result  of  44  powerful  thrusts,”  producing 
44  slices  ”  in  the  rock. 
The  definitions  and  descriptions  of  Prof.  Lapworth  and  the 
Geological  Survey  agree  precisely  with  those  employed  in  the 
present  series  of  papers.  They  apply  only  to  rock  in  the  solid  state. 
VII.  Discussion  of  the  Quesiion  whether  there  has  been  a 
Mixing  of  the  Original  Magmas. 
The  great  variations  of  chemical  composition  seen  in  the  Mal- 
vernian  rocks  have  suggested  to  some  correspondents  a  mixing  of 
the  acidic  and  basic  magmas  before  consolidation.  Perhaps  the  best 
reply  to  this  suggestion  is  set  forth  by  the  facts  contained  in  the 
present  series  of  papers.  There  is  evidence  to  prove  an  intermixture 
of  chemical  constituents,  that,  for  example,  potash  has  passed  from 
granite  to  diorite,  and  soda  from  diorite  to  granite.  But  I  have  never 
been  able  to  find  that  the  magmas  are  mixed  together,  except  perhaps 
1  4  Introductory  Text-book  of  Geology,’  1888,  p.  112. 
2  Quart.  Journ.  Geol.  Soc.  vol.  xliv.  (1888)  p.  378. 
