﻿282 
  Phillips 
  — 
  Minevalogical 
  Structure 
  and 
  Chemical 
  

  

  the 
  magma 
  lias 
  been 
  made 
  up 
  in 
  ferrous 
  iron 
  and 
  alkalis. 
  The 
  

   tendency 
  is, 
  then, 
  to 
  form 
  pyroxene 
  at 
  the 
  border, 
  rich 
  in 
  mag- 
  

   nesia, 
  of 
  the 
  composition 
  of 
  CaMgSijOe 
  when 
  the 
  magma 
  is 
  

   rich 
  in 
  magnesia, 
  or 
  of 
  the 
  composition 
  of 
  CaFeSi^Og 
  when 
  

   the 
  magma 
  is 
  rich 
  in 
  iron. 
  

  

  The 
  calcium 
  in 
  the 
  pyroxene 
  does 
  not 
  seem 
  subject 
  to 
  much 
  

   change. 
  

  

  There 
  has 
  been 
  an 
  increase 
  in 
  the 
  alkali 
  -bearing 
  molecule, 
  

   acmite, 
  as 
  the 
  amount 
  of 
  SiO^ 
  has 
  increased. 
  This 
  increase 
  of 
  

   the 
  acmite 
  molecule 
  as 
  the 
  acidity 
  of 
  the 
  magma 
  increases, 
  is 
  

   in 
  accord 
  with 
  the 
  experiments 
  of 
  F. 
  Becke.'^ 
  He 
  points 
  out 
  

   that 
  the 
  acmite 
  molecule 
  takes 
  part 
  in 
  the 
  composition 
  of 
  

   monoclinic 
  pyroxene, 
  crystallizing 
  from 
  magmas 
  with 
  a 
  per- 
  

   centage 
  of 
  SiO^ 
  of 
  50 
  or 
  above. 
  With 
  the 
  increase 
  of 
  6 
  per 
  

   cent 
  of 
  Si02 
  from 
  No. 
  2 
  to 
  No. 
  3 
  there 
  is 
  a 
  decided 
  increase 
  

   of 
  K^O 
  and 
  Na^O 
  found 
  in 
  the 
  pyroxene. 
  Campbell 
  and 
  

   Brown^ 
  have 
  shown 
  that 
  in 
  this 
  Mesozoic 
  diabase 
  rich 
  in 
  MgO, 
  

   where 
  it 
  reaches 
  an 
  amount 
  equal 
  to 
  12 
  per 
  cent 
  or 
  more, 
  the 
  

   orthorhombic 
  pyi'oxene 
  appears 
  as 
  the 
  mineral 
  hypersthene. 
  

   We 
  would 
  not 
  expect 
  an 
  orthorhombic 
  pyroxene 
  to 
  appear 
  in 
  

   the 
  Rocky 
  Hill 
  diabase, 
  as 
  the 
  proportion 
  of 
  (FeO+MgO) 
  : 
  CaO 
  

   in 
  the 
  analysis 
  show^ing 
  the 
  highest 
  percentages 
  of 
  these, 
  is 
  as 
  

   1-7:1, 
  a 
  proportion 
  of 
  FeO 
  and 
  MgO 
  far 
  below 
  the 
  ratio 
  at 
  

   which 
  it 
  has 
  been 
  found 
  necessary 
  for 
  these 
  to 
  be 
  present, 
  in 
  

   order 
  that 
  they, 
  in 
  crystallizing, 
  should 
  form 
  an 
  orthorhombic 
  

   pyroxene. 
  The 
  molecular 
  relation 
  of 
  (MgO 
  -f 
  FeO) 
  :CaO 
  must 
  

   be 
  as 
  3:1 
  or 
  greater.^" 
  This 
  high 
  proportion 
  .of 
  MgO 
  -H 
  FeO 
  

   is 
  only 
  locally 
  reached 
  in 
  this 
  diabase, 
  and 
  for 
  that 
  reason 
  it 
  is 
  

   to 
  be 
  expected 
  that 
  the 
  metasilicate 
  is 
  almost 
  invariably 
  some 
  

   form 
  of 
  the 
  monoclinic 
  pyroxene. 
  

  

  Analyses 
  of 
  Feldspars 
  from 
  Quarry 
  No. 
  2. 
  

  

  I. 
  

  

  

  II. 
  

  

  

  III. 
  

   West 
  Rock, 
  X.H.'i 
  

  

  lY. 
  

  

  Y. 
  

  

  Gravity 
  greater 
  

  

  

  

  Gravity 
  Gravity 
  

  

  Gravity 
  Gravity 
  

  

  than 
  2-69. 
  

  

  Gravity 
  < 
  

  

  2-69. 
  

  

  > 
  2-69. 
  <2-69. 
  

  

  2-703. 
  

  

  2-659. 
  

  

  SiO, 
  __. 
  53-84 
  

  

  •899 
  

  

  62-26 
  

  

  r037 
  

  

  52-84 
  60-54 
  

  

  53^7 
  

  

  62^ 
  

  

  Al^Og 
  .. 
  29-30 
  

  

  -287 
  

  

  21-87 
  

  

  •216 
  

  

  28-62 
  24-11 
  

  

  29^8 
  

  

  24- 
  

  

  FeP3.- 
  -81 
  

  

  •005 
  

  

  •54 
  

  

  •003 
  

  

  1-52 
  1-14 
  

  

  

  

  

  

  CaO- 
  -- 
  10-08 
  

  

  -180 
  

  

  6^53 
  

  

  -116 
  

  

  11-81 
  9-15 
  

  

  11-8 
  

  

  5^3 
  

  

  MgO 
  - 
  -28 
  

  

  -007 
  

  

  •15 
  

  

  -003 
  

  

  •46 
  -27 
  

  

  

  

  

  

  Na^O 
  .. 
  5-31 
  

  

  -085 
  

  

  7-98 
  

  

  -128 
  

  

  2^38 
  4-11 
  

  

  4^9 
  

  

  8-7 
  

  

  K,0... 
  1-16 
  

  

  -012 
  

  

  1^20 
  

  

  -012 
  

  

  •86 
  V06 
  

  

  

  

  

  

  Ignition 
  -44 
  

  

  — 
  

  

  •32 
  

   100^85 
  

  

  

  

  ro6 
  -59 
  

  

  '"" 
  

  

  

  

  101-22 
  

  

  99-75 
  100-97 
  

  

  

  I. 
  Gravity 
  > 
  2-69. 
  Ratio 
  of 
  

  

  

  

  

  

  R,0:RO:R 
  

  

  Ps 
  : 
  SiO 
  

  

  ^ 
  r= 
  -097 
  : 
  187 
  : 
  -292 
  

  

  •899. 
  

  

  

  