358  ON  AN  INTRUSION  OF  MUSCOVITE- BIOTlTE  GNEISS.  [Aug.  I  893, 
The  present  communication  showed  that  this  was  the  case.  He 
mentioned  a  letter  from  Mr.  Greenly,  who  stated  his  belief  that  great 
masses  of  gneiss  behave  as  igneous  rocks,  chiefly  as  sills,  and  that 
their  foliation  was  acquired  at  the  time  of  their  intrusion.  He 
(Mr.  Greenly)  had  observed  junctions  and  inclusions  of  schists  in 
gneiss  similar  to  those  figured  by  Lawson,  and  had  noticed  the  de¬ 
velopment  of  sillimanite  and  cyanite  in  such  of  the  sedimentary 
schists  as  could  have  furnished  material  for  the  growth  of  these 
minerals,, 
Hr.  Du  Riche  Preller  wished  to  ask  the  Author  whether  and 
how  far  the  more  recent  eruptive  rock  shown  in  the  diagram,  and 
marked  4  newer  diorite,’  had  affected  the  contiguous  older  igneous 
(granitic)  mass.  He  congratulated  the  Author  on  his  paper,  and  on 
the  ingenious  and  conclusive  way  in  which  he  had  worked  out  the 
process  of  the  potash-bearing  part  of  the  magma  being  forced  out, 
and  of  the  potash  being  subsequently  consolidated  and  crystallized 
into  microcline  and  muscovite.  In  this  connexion,  Dr.  Preller  asked 
what,  according  to  the  analysis  made,  was  the  relative  proportion  of 
silica  and  potash  in  the  rock  in  which  the  latter  formed  so  large  a 
constituent.  He  also  wished  to  express  his  entire  concurrence  in 
Dr.  Hicks’s  view  that  the  metamorphic  phenomena  described  in  the 
paper  were  due  not  only  to  thermal  but  also  to  dynamic  agency. 
Indeed,  it  could  not  be  otherwise,  for,  physically  speaking,  one 
agency  involved  the  other. 
Mr.  Teall  said  he  had  watched  the  growth  of  this  paper,  and 
could  testify  to  the  great  amount  of  important  detail  contained  in  it. 
It  was  almost  impossible  to  give  an  idea  of  such  a  paper  in  the  short 
time  available  for  reading.  Vernadsky  had  shown  that  cyanite 
could  be  converted  into  sillimanite  by  heating  to  about  1300° 
centigrade,  and  the  latter  mineral  had  been  proved  by  Mr.  Barrow 
to  occur  in  the  most  highly  altered  area.  It  appeared  probable, 
therefore,  that  the  line  separating  the  sillimanite-zone  from  tho 
cyanite-zone  was  an  isothermal. 
The  Author  only  claimed  credit  for  holding  fast  to  Lyell’s 
principles  of  uniformitarianism.  In  answer  to  the  President,  he 
said  that  the  metamorphism  of  the  newer  granites  could  be  easily 
separated  from  that  which  he  had  described.  Its  effect  was  to 
destroy  many  of  the  characters  due  to  the  earlier  action. 
In  reply  to  Dr.  Preller,  he  remarked  that  the  microcline  of  the 
pegmatites  was  perfectly  normal  in  its  composition,  but  that 
owing  to  the  coarseness  of  the  grain  he  had  not  determined  the 
relative  proportions  of  felspar  and  quartz. 
