﻿1S2 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  amount 
  of 
  the 
  dark 
  colored 
  mica 
  mixed 
  with 
  the 
  feldspar 
  and 
  

   quartz; 
  and 
  the 
  dark 
  colored 
  varieties 
  owe 
  their 
  color, 
  in 
  most 
  

   cases, 
  to 
  hornblende 
  or 
  tourmaline 
  which 
  may 
  be 
  present. 
  

  

  The 
  beauty, 
  ease 
  of 
  working, 
  durability 
  and 
  value 
  of 
  the 
  gran- 
  

   ites 
  for 
  use 
  in 
  construction 
  is 
  related 
  closely 
  to 
  their 
  mineralo- 
  

   gical 
  composition. 
  Their 
  arrangement 
  in 
  the 
  mass 
  and 
  their 
  

   relative 
  proportions 
  determine 
  the 
  color 
  and 
  give 
  beaut3\ 
  The 
  

   presence 
  or 
  absence 
  of 
  certain 
  species 
  influence 
  the 
  hardness 
  and 
  

   homogeneous 
  nature 
  and 
  the 
  consequent 
  ease 
  with 
  which 
  the 
  

   stone 
  can 
  be 
  dressed 
  and 
  polished. 
  For 
  example 
  the 
  mica, 
  if 
  

   disposed 
  in 
  parallel 
  surfaces, 
  gives 
  a 
  foliated 
  structure 
  and 
  tends 
  

   to 
  produce 
  what 
  is 
  known 
  as 
  rift, 
  and 
  the 
  granite 
  is 
  more 
  readily 
  

   split 
  in 
  the 
  planes 
  of 
  the 
  mica 
  than 
  across 
  them. 
  Again 
  the 
  mica 
  

   flakes 
  may 
  be 
  so 
  large 
  and 
  irregularly 
  massed 
  that 
  the 
  surface 
  is 
  

   not 
  susceptible 
  of 
  a 
  uniform 
  degree 
  of 
  polish. 
  Hornblende, 
  on 
  

   account 
  of 
  its 
  superior 
  toughness, 
  is 
  less 
  brittle 
  than 
  pyroxene 
  

   under 
  the 
  polishing, 
  and 
  the 
  hornblende 
  granites 
  are 
  said 
  to 
  be 
  

   preferred 
  to 
  those 
  rocks 
  which 
  contain 
  pyroxene 
  in 
  quantity. 
  

  

  The 
  more 
  nearly 
  alike 
  in 
  hardness 
  and 
  the 
  more 
  intimately 
  

   interwoven 
  the 
  texture 
  of 
  the 
  minerals, 
  the 
  more 
  capable 
  they 
  are 
  

   of 
  receiving 
  a 
  good 
  polish. 
  Hence 
  it 
  follows 
  that 
  the 
  very 
  coarse 
  

   crystalline 
  granites 
  are 
  not 
  so 
  well 
  suited 
  for 
  ornamental 
  work. 
  

  

  The 
  enduring 
  properties 
  of 
  granites 
  vary 
  with 
  the 
  nature 
  of 
  

   the 
  minerals 
  in 
  their 
  composition. 
  Although 
  popularly 
  they 
  are 
  

   regarded 
  as 
  our 
  most 
  durable 
  building 
  stone, 
  there 
  are 
  some 
  

   notable 
  exceptions, 
  which 
  are 
  evident 
  in 
  the 
  natural 
  outcrops, 
  

   where 
  this 
  rock 
  is 
  found 
  decayed 
  to 
  the 
  depth 
  of 
  100 
  to 
  200 
  feet, 
  

   and 
  in 
  the 
  active 
  disintegration 
  which 
  is 
  in 
  progress 
  in 
  structures 
  

   of 
  the 
  present 
  century. 
  Foliated 
  varieties 
  placed 
  on 
  edge 
  in 
  

   buildings, 
  tend 
  necessarily 
  to 
  scale 
  under 
  the 
  great 
  changes 
  of 
  

   temperature 
  in 
  our 
  northern 
  cities 
  and 
  towns. 
  The 
  more 
  rapid 
  

   decomposition 
  of 
  the 
  micas 
  makes 
  those 
  varieties 
  in 
  which 
  they 
  

   occur 
  in 
  large 
  flakes 
  or 
  aggregations 
  more 
  liable 
  to 
  decay. 
  The 
  

   condition 
  of 
  the 
  feldspar 
  also 
  is 
  often 
  such 
  as 
  to 
  influence 
  the 
  

   durability. 
  When 
  kaolinized 
  in 
  part, 
  it 
  is 
  an 
  element 
  of 
  weak- 
  

   ness 
  rather 
  than 
  of 
  strength. 
  The 
  presence 
  of 
  the 
  easily 
  decom- 
  

  

  