﻿672 
  Forty-seventh 
  Report 
  ox 
  the 
  State 
  Museum. 
  

  

  and 
  gneiss, 
  between 
  gneiss 
  and 
  Potsdam 
  and 
  between 
  

   gabbro 
  and 
  Potsdam 
  are 
  well 
  shown. 
  The 
  area 
  abounds 
  

   in 
  dikes, 
  which 
  will 
  be 
  mapped 
  and 
  studied 
  at 
  the 
  earliest 
  

   opportunity. 
  The 
  encircling 
  gneiss 
  is 
  very 
  variable 
  in 
  character, 
  

   and 
  passes 
  in 
  places 
  into 
  highly 
  micaceous 
  biotite 
  schist. 
  The 
  

   major 
  portion, 
  however, 
  is 
  of 
  the 
  ordinary 
  acidic 
  gneiss. 
  

   The 
  gabbro 
  in 
  large 
  part 
  presents 
  a 
  distinct 
  gneissic 
  structure, 
  

   the 
  dark 
  bisilicates 
  being 
  in 
  parallel 
  bands 
  which, 
  alternating 
  

   with 
  broader 
  bands 
  of 
  white 
  or 
  greenish 
  white 
  labradorite, 
  give 
  

   the 
  rock 
  a 
  striking 
  appearance. 
  Near 
  Mr. 
  Sanger's 
  the 
  rock 
  

   more 
  nearly 
  approaches 
  anorthosite 
  than 
  in 
  any 
  other 
  exposure 
  

   seen, 
  and 
  lacks 
  the 
  gneissic 
  structure. 
  Much 
  of 
  it 
  exposed 
  

   elsewhere 
  is 
  quite 
  basic, 
  and 
  this 
  is 
  notably 
  true 
  as 
  the 
  periphery 
  

   of 
  the 
  area 
  is 
  approached, 
  where 
  the 
  rock 
  is 
  in 
  general 
  quite 
  

   dark 
  colored. 
  The 
  great 
  abundance 
  of 
  garnet 
  at 
  nearly 
  all 
  

   exposures 
  is 
  a 
  noteworthy 
  characteristic 
  and 
  one 
  of 
  the 
  

   many 
  indications 
  of 
  the 
  great 
  alteration 
  that 
  the 
  rock 
  has 
  

   undergone. 
  The 
  encircling 
  Potsdam 
  consists 
  of 
  arkose 
  instead 
  

   of 
  sandstone, 
  and 
  consequently 
  readily 
  decays 
  and 
  disintegrates, 
  

   so 
  that 
  its 
  outcrops 
  are 
  not 
  prominent. 
  The 
  single 
  slide 
  so 
  far 
  

   prepared 
  shows 
  that 
  nearly 
  one-half 
  of 
  the 
  rock 
  is 
  made 
  up 
  

   of 
  feldspathic 
  grains, 
  the 
  feldspar 
  being 
  mainly 
  orthoclase, 
  a 
  

   small 
  amount 
  of 
  microcline 
  being 
  also 
  present 
  with 
  an 
  occasional 
  

   grain 
  of 
  plagioclase. 
  

  

  Potsdam 
  sandstone. 
  — 
  The 
  Potsdam 
  is 
  much 
  the 
  most 
  wide- 
  

   spread 
  formation 
  in 
  the 
  area 
  mapped, 
  its 
  width 
  of 
  outcrop 
  north 
  

   of 
  the 
  Adirondacks 
  being 
  very 
  considerable. 
  No 
  data 
  are 
  yet 
  

   at 
  hand 
  for 
  determining 
  its 
  thickness, 
  though 
  measurements 
  of 
  

   several 
  hundred 
  feet, 
  embodying 
  only 
  the 
  known 
  portion 
  of 
  

   the 
  whole 
  mass, 
  have 
  been 
  made, 
  as 
  for 
  instance 
  that 
  by 
  Mr. 
  Wal- 
  

   cott 
  at 
  Ausable 
  Chasm, 
  on 
  the 
  south 
  line 
  of 
  the 
  county.* 
  

   Moreover 
  it 
  lies 
  un 
  conformably 
  on 
  the 
  earlier 
  rocks, 
  so 
  that 
  its 
  

   thickness 
  is 
  a 
  variable 
  quantity. 
  It 
  presents 
  great 
  variation 
  in 
  

   coarseness, 
  massiveness, 
  color 
  and 
  degree 
  of 
  induration. 
  Many 
  

   layers 
  disintegrate 
  rapidly 
  ; 
  others 
  approach 
  quartzite 
  in 
  appear- 
  

   ance 
  and 
  durability 
  ; 
  much 
  of 
  that 
  forming 
  the 
  Flat 
  Rocks 
  in 
  

   Altona 
  is 
  quite 
  pebbly, 
  but 
  elsewhere 
  in 
  the 
  district 
  a 
  coarse 
  grit 
  

   is 
  the 
  main 
  phase; 
  occasionally 
  it 
  becomes 
  quite 
  thin-bedded 
  and 
  

  

  * 
  Bull. 
  U. 
  S. 
  Geol. 
  Surv., 
  No. 
  80, 
  p. 
  344. 
  

  

  