538  PBOF.  J.  W.  JTTDD  OX  COMPOSITE  DYKES  IX  ARK  AX.  [NoV.  1 893, 
composite  dykes  of'  both  classes  must  be  regarded  as  having,  at  the 
present  time,  a  special  interest. 
The  first  class  of  composite  dykes — including  those  in  which 
differentiation  has  clearly  gone  on  within  the  dyke  itself — has  been 
treated  of  with  much  ability  in  two  very  interesting  memoirs  that 
have  recently  appeared.  Dr.  A.  C.  Lawson  has  described  the  great 
diabase  dykes  that  intersect  the  Archaean  rocks  of  the  Rainy  Lake 
region  in  Canada.  By  the  aid  of  a  series  of  chemical  analyses  made 
by  Mr.  F.  T.  Shutt,  Dr.  Lawson  has  been  able  to  show  how  great  an 
amount  of  differentiation  has  sometimes  taken  place  within  the  very 
wide  dykes  of  that  region ;  and  the  various  rocks  thus  produced 
have  been  made  the  subject  of  careful  microscopic  study.1 
In  the  Stop  Island  Dyke,  150  feet  wide,  the  sides  of  the  dyke  are 
shown  to  present  the  characters  of  an  altered  augite-andesite  (por- 
phyrite)  and  to  contain  47-8  per  cent,  of  silica.  Tracing  the  rock 
towards  the  centre  of  the  dyke,  it  is  found  to  pass  first  into  an 
ophitic  dolerite  (diabase),  and  finally  into  a  uralitic  quartz-gabbro, 
containing  57-5  per  cent,  of  silica, — a  rock  in  which  various  acces¬ 
sory  minerals  are  much  more  abundant  than  in  the  rock  of  the  sides 
of  the  dyke. 
The  Whitefish  Bay  dyke,  120  feet  wide,  is  at  its  sides  composed 
of  fine-grained  ophitic  rock  (diabase)  with  47’5  per  cent,  of  silica ; 
but  it  passes,  towards  the  centre,  into  a  rock  which  is  partially 
granular  or  granitic  in  texture,  and  has  a  silica-percentage  of  52*5. 
Hvpersthene,  which  is  abundant  at  the  sides  of  the  dyke,  disappears 
in  the  central  part,  while  quartz  and  hornblende  come  in  instead. 
Similar  differentiation  of  the  materials  within  a  fissure  is  shown 
by  Dr.  Lawson  to  have  gone  on  in  many  other  dykes  in  the  district. 
As  a  general  rule,  the  centres  of  the  dykes  are  richer  in  silica  than 
the  sides ;  and  not  only  is  there  a  difference  in  the  degree  of 
development  of  crystalline  structure,  so  that  a  ‘  trachytoid  ’  or 
4  ophitic  5  rock  at  the  sides  passes  into  a  perfectly  holocrystalline 
(granitic)  rock  at  the  centre,  but  certain  minerals,  like  enstatite, 
which  abound  at  the  sides  disappear  in  the  middle  of  the  dyke, 
being  replaced  by  others,  such  as  quartz,  hornblende,  and  garnet. 
Dr.  Lawson  has  also  called  attention  to  the  curious  circumstance  that 
the  crystals  of  the  same  mineral  may  show  marked  differences  in 
susceptibility  to  chemical  change,  when  they  are  found  developed  in 
different  parts  of  the  same  dyke. 
Even  more  striking  than  the  examples  of  composite  dykes  from 
Canada  is  the  case  of  the  Huk  dyke  of  Norway,  recently  described 
by  Prof.  I.  H.  L.  Yogt.2 * *  This  dyke  occurs  in  the  Christiania 
1  ‘  Petrographical  Differentiation  of  Certain  Dykes  of  the  Rainy  Lake 
Region,’  bv  Andrew  0.  Lawson,  with  Analyses  by  F.  T.  Shutt,  ‘  American 
Geologist,’  voLvii.  (1891)  pp.  153-164 ;  see  also  ‘  Notes  on  some  Diabase  Dykes 
of  the  Rainv  Lake  Region,’  Froc.  Canad.  Inst.  1887,  ser.  3,  vol.  v.  p.  173,  and 
‘  Report  on  the  Geology  of  the  Rainy  Lake  Region,’  by  the  same  author,  Geol. 
&  Nat.  Hist.  Surv.  Canada,  Annual  Report,  1887—88,  part  F. 
2  ‘  Om  Dannelsen  af  de  vigtigste  i  Norge  og  Sverige  represen terede  Crupper 
af  JernmalmforekomsteiV  af  J.  H.  L.  Vogt,  Geologisk.  Forening.  Forkandl. 
vol.  xiii.  (1891)  pp.  476  &  683,  and  vol.  xiv.  (1892)  p.  211  ;  see  also  Geol.  Mag. 
for  1892,  pp.  82-86,  and  Zeitsckr.  f.  prakt.  Geol.  vol.  i.  (1892)  p.  4. 
