﻿250 
  GEOLOGY. 
  

  

  The 
  quartz 
  pebbles, 
  on 
  the 
  contrary, 
  whether 
  of 
  the 
  

   limpid, 
  milky, 
  or 
  ferruginous 
  varieties, 
  uniformly 
  occur 
  

   in 
  symmetrical 
  convex 
  disks, 
  sometimes 
  nearl}^ 
  circular, 
  

   but 
  most 
  generally 
  of 
  an 
  ovate 
  form. 
  

  

  The 
  carnelians 
  vary 
  in 
  color 
  from 
  the 
  brown 
  or 
  sards 
  

   to 
  the 
  white 
  or 
  chalcedony, 
  but 
  are 
  generally 
  of 
  an 
  

   amber 
  or 
  pale 
  red, 
  and 
  rarely 
  afford 
  a 
  facet 
  over 
  an 
  inch 
  

   square, 
  although 
  occurring 
  sometimes 
  three 
  or 
  four 
  times 
  

   as 
  large. 
  

  

  The 
  sards 
  are 
  generally 
  smooth, 
  flat, 
  and 
  present 
  a 
  

   much 
  larger 
  surface, 
  possessing 
  a 
  somewhat 
  conchoidal 
  

   fracture, 
  and 
  the 
  edges 
  are 
  generally 
  of 
  unequal 
  thick- 
  

   ness. 
  

  

  The 
  finer 
  varieties 
  of 
  agates 
  are 
  of 
  a 
  closer 
  texture, 
  

   and 
  freer 
  from 
  flaws 
  than 
  the 
  carnelians, 
  being 
  of 
  the 
  

   composition 
  of 
  chalcedony, 
  striped 
  with 
  variouslj-colored 
  

   veins, 
  or 
  concentric 
  rings, 
  and 
  sometimes 
  clouded, 
  and 
  

   are 
  known 
  as 
  sard 
  agates, 
  or 
  sardonyx, 
  in 
  contradistinc- 
  

   tion 
  to 
  the 
  larger 
  and 
  coarser 
  kinds 
  first 
  mentioned, 
  in 
  

   which 
  the 
  crystalline 
  quartz 
  and 
  jasper 
  are 
  generally 
  

   combined 
  in 
  variable 
  proportions. 
  

  

  The 
  sard 
  agates 
  are 
  fully 
  equal 
  to 
  the 
  German 
  or 
  ori- 
  

   ental 
  agate 
  in 
  beauty 
  and 
  texture. 
  

  

  All 
  these 
  varieties 
  are 
  susceptible, 
  in 
  the 
  hands 
  of 
  the 
  

   lapidary, 
  of 
  being 
  formed 
  into 
  handsome 
  gems. 
  Many 
  

   of 
  them 
  have 
  been 
  cut 
  and 
  polished, 
  and 
  are 
  much 
  ad- 
  

   mired. 
  

  

  As 
  these 
  pebbles 
  have 
  all 
  been 
  subjected 
  to 
  an 
  equal 
  

   degree 
  of 
  attrition, 
  the 
  ultimate 
  variety 
  of 
  form 
  is 
  doubt- 
  

   less 
  owing 
  to 
  their 
  distinctive 
  crystalline 
  structure. 
  

  

  In 
  illustration 
  of 
  the 
  degree 
  of 
  force 
  necessarily 
  em- 
  

   ployed 
  in 
  the 
  transportation 
  of 
  the 
  materials 
  comprising 
  

   the 
  drift, 
  it 
  is 
  proper 
  perhaps 
  to 
  speak 
  more 
  definitely 
  of 
  

   their 
  dimensions. 
  

  

  