﻿Geology 
  and 
  Natural 
  History. 
  383 
  

  

  broad 
  undulations, 
  but 
  are 
  crowded 
  over 
  at 
  the 
  west 
  into 
  a 
  some- 
  

   what 
  sharp 
  anticline." 
  In 
  the 
  Cambrian 
  slate 
  mass 
  there 
  are 
  

   close 
  folds 
  more 
  or 
  less 
  overturned 
  to 
  the 
  west 
  with 
  easterly 
  dip- 
  

   ping 
  slaty 
  cleavage 
  obscuring 
  the 
  bedding. 
  " 
  The 
  folds 
  are 
  so 
  

   close 
  and 
  the 
  cleavage 
  is 
  so 
  pronounced 
  that 
  the 
  cores 
  of 
  adjoin- 
  

   ing 
  synclines 
  and 
  anticlines 
  are 
  brought 
  very 
  near 
  together 
  or 
  

   the 
  anticlinal 
  portions 
  of 
  several 
  folds 
  do 
  not 
  appear. 
  Series 
  of 
  

   various 
  folds 
  form 
  compound 
  anticlines 
  and 
  these 
  minor 
  Cambrian 
  

   anticlinoria 
  alternate 
  with 
  Ordovician 
  synclinoria 
  conformably 
  

   overlying 
  Cambrian 
  ones." 
  The 
  details 
  of 
  structure 
  along 
  these 
  

   sections 
  — 
  bedding, 
  cleavage, 
  grain, 
  etc. 
  are 
  fully 
  discussed 
  and 
  

   illustrated 
  by 
  numerous 
  photographs 
  and 
  diagrams. 
  

  

  Much 
  attention 
  has 
  been 
  given 
  in 
  this 
  paper 
  to 
  the 
  chemical 
  

   and 
  mineralogical 
  composition 
  of 
  the 
  slates. 
  Complete 
  analyses 
  

   have 
  been 
  made 
  of 
  seven 
  varieties 
  and 
  the 
  mineralogical 
  composi- 
  

   tion 
  of 
  the 
  various 
  types 
  has 
  been 
  determined 
  by 
  careful 
  exam- 
  

   ination 
  of 
  over 
  200 
  thin 
  sections. 
  In 
  addition 
  to 
  the 
  microscopic 
  

   descriptions, 
  five 
  colored 
  lithographic 
  representations 
  of 
  mi- 
  

   croscopic 
  sections 
  are 
  given 
  of 
  each 
  variety 
  of 
  commercial 
  

   importance. 
  These 
  illustrate 
  clearly 
  the 
  differences 
  in 
  texture 
  

   and 
  composition 
  which 
  underlie 
  differences 
  in 
  quality 
  and 
  color. 
  

  

  Under 
  the 
  head 
  of 
  Scientific 
  Geology 
  (pp. 
  278-300) 
  Prof. 
  Dale 
  

   deals 
  with 
  the 
  more 
  purely 
  scientific 
  problems 
  connected 
  with 
  

   slates 
  in 
  general 
  and 
  those 
  of 
  the 
  New 
  York-Vermont 
  region 
  in 
  

   particular. 
  He 
  first 
  reviews 
  the 
  present 
  state 
  of 
  science 
  on 
  the 
  

   chemical 
  and 
  physical 
  constitution 
  and 
  origin 
  of 
  slates. 
  This 
  criti- 
  

   cal 
  discussion 
  taken 
  together 
  with 
  the 
  bibliography 
  (pp. 
  166-174) 
  

   is 
  especially 
  valuable 
  lor 
  students 
  in 
  that 
  it 
  brings 
  together 
  in 
  an 
  

   American 
  publication 
  a 
  synopsis 
  of 
  all 
  that 
  is 
  known 
  about 
  

   "slate." 
  Several 
  pages 
  are 
  next 
  devoted 
  to 
  the 
  complex 
  question 
  

   of 
  the 
  relations 
  between 
  the 
  Lower 
  Cambrian 
  and 
  the 
  Lower 
  

   Ordovician 
  in 
  this 
  region. 
  Indications 
  of 
  slight 
  unconformity 
  

   between 
  these 
  horizons 
  are 
  noted 
  and 
  the 
  probability 
  of 
  local 
  

   diversity 
  of 
  contemporaneous 
  sedimentation 
  is 
  explained. 
  The 
  

   geological 
  history 
  of 
  the 
  slate 
  belt 
  is 
  the 
  last 
  subject 
  of 
  a 
  theoret- 
  

   ical 
  nature 
  treated. 
  In 
  brief 
  the 
  history 
  is 
  as 
  follows 
  : 
  A 
  land 
  

   surface 
  chiefly 
  of 
  granites 
  and 
  gneisses 
  was 
  covered 
  by 
  the 
  sea 
  in 
  

   w"hich 
  were 
  deposited 
  Lower 
  Cambrian 
  sediments 
  which 
  show 
  

   by 
  their 
  character 
  great 
  variations 
  in 
  conditions 
  of 
  deposition. 
  

   For 
  some 
  reason 
  no 
  Middle 
  and 
  Upper 
  Cambrian 
  deposits 
  

   were 
  laid 
  down, 
  but 
  grits, 
  shales 
  and 
  slates 
  of 
  the 
  Ordovician 
  

   follow 
  the 
  Lower 
  Cambrian 
  directly. 
  At 
  the 
  close 
  of 
  the 
  

   Ordovician, 
  the 
  accumulated 
  sediments, 
  both 
  Cambrian 
  and 
  

   Ordovician, 
  gradually 
  emerged 
  in 
  folds 
  more 
  intensely 
  plicated 
  

   toward 
  the 
  east. 
  The 
  increasing 
  pressure 
  produced 
  metamor- 
  

   phism 
  and 
  percolating 
  alkaline 
  waters 
  deposited 
  silica 
  in 
  open- 
  

   ings 
  formed 
  by 
  the 
  folding. 
  These 
  movements 
  produced 
  the 
  

   slates. 
  At 
  some 
  later 
  time 
  another 
  movement 
  occurred 
  resulting 
  

   in 
  diagonal 
  joints, 
  quartz 
  veins, 
  shear 
  zones 
  and 
  other 
  secondary 
  

   structures. 
  Some 
  of 
  the 
  fractures 
  were 
  so 
  deep 
  as 
  to 
  occasion 
  

  

  