384  CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1906,  PART    I. 
1 1  ie  folding.  In  the  more  quartzose  layers,  however,  secondary  fissility 
has  been  developed  but  weakly  if  at  all,  and  such  layers  are  in  general! 
numerous  enough  to  define  the  trend  of  the  original  bedding  and  to] 
render  this  the  direction  of  easiest  parting  so  far  as  the  intrusion  on 
considerable  masses  of  igneous  rock  is  concerned.  Over  large  areas 
the  rocks  have  been  thrown  into  a  series  of  closely  compressed  (iso- 
clinal) folds  standing  in  highly  inclined  positions,  and  in  such  areas 
the  secondary  fissility  is  as  a  rule  nearly  parallel  to  the  bedding  planes. 
The  form  of  the  pegmatite  bodies,  which  in  their  intrusion  followed  in! 
general  the  direction  of  least  resistance,  is  therefore  dependent  largely 
on  the  position  of  the  bedding  planes  in  the  surrounding  sedimentary 
strata.  Their  form  in  regions  of  highly  inclined  strata  is  well  shown 
along  the  shore  near  Boothbay  Harbor,  where  most  of  the  pegmatite 
masses  are  highly  inclined  and  dikelike  in  form,  though  differing  from 
the  many  dikes  of  diabase  and  of  normal  granite  of  the  same  region 
in  not  showing  parallel  walls,  the  typical  form  being  a  succession  of 
lenticular  masses  produced  by  repeated  pinchings  and  swellings  of  the 
dike.  In  regions  where  the  sedimentary  strata  are  but  slightly 
inclined  the  pegmatite  masses  are  lint-lying  and  sill-like  rather  than 
dikelike  in  form,  though  showing  the  same  tendency  toward  lenticular 
form  or  toward  repeated  pinchings  and  swellings  along  the  length  of 
the  sill.  On  account  of  their  flat-lying  character  many  pegmatite 
bodies  of  the  latter  type  cover  considerable  areas  and  show  a  rather 
irregular  surface  outcrop.  ( )ther  bodies  are  exposed  only  in  cross  seel 
tion  in  a  quarry  cut  or  along  the  side  of  a  valley.  One  of  the  best 
examples  of  the  flat-lying  pegmatite  masses  is  exposed  in  the  bed  of 
Androscoggin  River  jusl  above  the  road  bridge  bet  ween  Lewiston  and 
Auburn.  The  inclosing  rocks  here  are  gray  to  purplish  slates  thai 
show  distinct  bedding  and  dip  to  the  northeast  at  angles  of  about  30°j 
and  several  sill-like  masses  of  pegmatite  are  intruded  parallel  to  their 
bedding  planes.  The  largest  mass  is  lens-shaped,  with  a  maximum 
thickness  of  1  5  feet ,  and  is  exposed  laterally  for  300  feet,  though  prob- 
ably extending  much  farther.  The  position  of  the  falls  here  is  un- 
doubtedly dependent  on  the  fact  that  this  sill  and  the  injected  sedi- 
ments adjacent  to  it  offer  more  resistance  to  erosion  than  most  of  the 
schists.  The  pegmatite  body  that  is  worked  for  its  gem  minerals  at 
Mount  Mica,  in  Paris,  Oxford  County,  is  another  example  of  the  flat- 
lying,  sill-like  type.  The  sedimentary  schists  here  dip  20°  to  30°  SW. 
and  the  pegmatite  mass,  apparently  20  feet  or  so  in  thickness,  has  a 
similar  dip.  Other  pegmatite  masses  are  unlike  either  of  the  types 
just  described,  but  seem  to  be  large,  somewhat  irregular  stocks  of  uni- 
form character  throughout.  One  of  the  best  examples  of  this  type  is j 
Streaked  Mountain,  in  the  northwest  corner  of  the  town  of  Hebron, 
Oxford  County,  which  is  almost  wholly  pegmatite  and  seems  to  be  ai 
great  dome  of  this  rock. 
