418  DR.  c.  CALLAWAY  ON  TELE  ORIGIN  OF  THE  [Aug.  1 893, 
occasionally  on  a  small  scale  at  their  contacts.  We  have  seen  that 
the  grey  diorite  was  injected  into  the  medium-black  variety,  that 
granite,  felsite,  and  dolerite  were  subsequently  intruded  into  the 
two  preceding  ;  yet  each  of  these  magmas  is  nearly  always  clearly 
and  sharply  distinguishable  from  the  others. 
The  variations  of  chemical  composition  seen  within  the  same 
rock-mass  depend  largely  upon  two  causes — namely,  the  degree  of 
mechanical  energy,  and  the  nature  and  quantity  of  infiltrated 
mineral  matter.  In  numerous  cases  chemical  change  is  seen  to 
increase  pari  passu  with  the  mechanical  force,  as  in  the  conversion 
of  diorite  into  sericite-gneiss  at  Ragged  Stone  Hill  (pp.  404  et  seqq.). 
The  influence  of  infiltration  is  well  illustrated  at  Swinyard's  Hill 
(p.  413),  where,  as  we  trace  the  diorite  into  the  granite-diorite 
complex,  the  percentage  of  potash  in  the  diorite  rises  from  2-8  to  5-3. 
VIII.  The  Chemical  Changes. 
The  Elimination  of  Magnesia. — Most  of  the  decompositions  de¬ 
scribed  in  these  papers  are  well  known.  The  action  of  carbonic 
acid  and  alkaline  carbonates,  at  the  ordinary  temperature  and  pres¬ 
sure,  upon  silicates  of  the  heavy  bases  was  described  by  Bischof. 
He  held  that  in  his  experiments  silicate  of  lime  and  protosilicate 
of  iron  were  decomposed  by  carbonic  acid  in  the  wet  way,  but  that 
silicate  of  magnesia  was  only  slightly  acted  upon,  and  silicate  of 
alumina  not  at  all.  He  admits,  however,  that  in  nature  silicate  of 
magnesia  is  decomposed  by  carbonic  acid,  even  at  ordinary  tempera¬ 
tures.  Thus,  he  says  that  in  springs  carbonate  of  magnesia  has 
been  formed  by  the  decomposition  of  magnesian  silicates  by 
carbonic  acid.  Again,  in  the  conversion  of  olivine  into  serpentine 
magnesia  is  eliminated,  and  Bischof  quotes  a  case  said  to  occur  at 
Snarum,  where  the  serpentine  thus  produced  is  surrounded  by 
bitter-spar.  He  also  states  that  dolomite  has  been  observed  in 
limestone  where  it  was  in  contact  with  basalt,  the  change  being 
produced  by  the  action  of  meteoric  waters. 
The  researches  of  Mr.  Alexander  Johnstone,  F.G.S.,  have  an 
important  bearing  upon  this  enquiry.  He  concludes  1  that  water 
containing  carbonic  acid  exerts  no  chemical  change  upon  pure 
mineral  magnesian  silicates ;  but  that,  even  at  the  ordinary  tem¬ 
peratures,  carbonated  water  extracts  magnesia  from  complex  sili¬ 
cates,  such  as  olivine.  This  mineral  was  exposed  for  two  months 
to  the  action  of  distilled  water  saturated  with  carbon  dioxide.  The 
liquid  was  then  found  to  contain  magnesia,  and  the  olivine  had  lost 
*73  per  cent,  of  that  base.  Mr.  Johnstone  exposed  biotite-mica 
dust  to  carbonic-acid  water  for  a  year.  He  states 2  that,  at  the 
end  of  that  time,  the  percentage  of  magnesia  had  been  reduced 
Torn  20-04  to  17‘35  ;  and  he  suggests  that  an  extension  of  the 
t  me  of  immersion  would  result  in  the  further  elimination  of 
the  base.  Rrom  a  long  series  of  experiments,  he  concludes  that 
1  Proc.  Roy.  Soc.  Edin.  vol.  xv.  (1888)  p.  436. 
2  Quart.  Journ.  Geol.  Soc.  vol.  xlv.  (1889)  p.  367. 
