﻿Geology. 
  427 
  

  

  centrated 
  even 
  to 
  an 
  abundant 
  deposition 
  of 
  common 
  salt. 
  On 
  

   account 
  of 
  the 
  minuteness 
  of 
  the 
  crystals, 
  he 
  was 
  not 
  able 
  posi- 
  

   tively 
  to 
  prove 
  their 
  rhombohedral 
  character, 
  but 
  believes 
  that 
  

   there 
  is 
  little 
  doubt 
  of 
  this. 
  The 
  author 
  deduces 
  from 
  his 
  

   experiments, 
  that 
  the 
  massive 
  dolomite 
  limestones 
  have 
  been 
  

  

  oduced 
  in 
  closed 
  lagoons 
  along 
  a 
  sea 
  bottom 
  through 
  the 
  

  

  3 
  

  

  P 
  

  

  action 
  of 
  the 
  sea-water, 
  concentrated 
  by 
  the 
  sun's 
  heat, 
  on 
  coral- 
  

   made 
  sediments, 
  conforming, 
  as 
  he 
  says, 
  to 
  Dana's 
  views 
  on 
  

   Dolomization. 
  

  

  It 
  should 
  be 
  here 
  understood 
  that 
  Dana- 
  has 
  never 
  supposed 
  

   that 
  the 
  dolomized 
  calcareous 
  sediments 
  were 
  always, 
  or 
  

   generally, 
  those 
  of 
  coral 
  reef 
  origin; 
  but 
  that 
  any 
  calcareous 
  

   sediments, 
  whether 
  from 
  shells 
  or 
  corals, 
  or 
  any 
  other 
  source, 
  

   would 
  undergo 
  the 
  chemical 
  change, 
  under 
  the 
  conditions 
  stated. 
  

   It 
  is 
  important, 
  therefore, 
  to 
  note 
  that 
  the 
  shells 
  of 
  most 
  Gastro- 
  

   pods 
  and 
  Cephalopods, 
  and 
  the 
  inner 
  pearly 
  layer 
  of 
  many 
  

   Lamellibranchs, 
  are 
  aragonite 
  ; 
  and 
  that 
  such 
  shells 
  contribute 
  to 
  

   the 
  material 
  of 
  coral 
  reef 
  rocks, 
  as 
  well 
  as 
  to 
  ordinary 
  lime- 
  

   stones. 
  J. 
  D. 
  D. 
  

  

  3. 
  On 
  Dolomite- 
  making 
  and 
  dolomitic 
  calcareous 
  organisms. 
  — 
  

   Under 
  this 
  title, 
  A. 
  G. 
  Hogbom, 
  of 
  Stockholm, 
  discusses 
  (Jahrb. 
  f. 
  

   Miu.,Geol. 
  undPal., 
  1894, 
  vol. 
  i,p. 
  272), 
  the 
  question 
  as 
  to 
  the 
  origin 
  

   of 
  the 
  magnesia 
  of 
  dolomitic 
  limestones. 
  He 
  finds 
  by 
  analysis 
  

   that 
  the 
  common 
  incrusting 
  Nullipores 
  of 
  coral 
  reefs 
  — 
  species 
  of 
  

   the 
  genus 
  Lithothamnion 
  — 
  commonly 
  contain 
  a 
  large 
  percentage 
  

   of 
  magnesium 
  carbonate. 
  He 
  gives 
  the 
  results 
  of 
  fourteen 
  

   analyses 
  — 
  1 
  by 
  himself, 
  9 
  by 
  X. 
  Sahlbom, 
  3 
  by 
  R. 
  Mauzelius, 
  

   and 
  1 
  by 
  J. 
  Guinchard, 
  and 
  he 
  deduces 
  for 
  the 
  mean 
  of 
  the 
  

   whole, 
  10 
  parts 
  of 
  magnesium 
  carbonate 
  to 
  100 
  of 
  calcium 
  

   carbonate. 
  

  

  Mr. 
  Hogbom 
  also 
  cites 
  results 
  from 
  analysis 
  of 
  Bermuda 
  

   corals, 
  by 
  G. 
  Forsstrand, 
  in 
  which 
  the 
  amount 
  of 
  magnesium 
  

   carbonate 
  and 
  calcium 
  carbonate, 
  was, 
  for 
  a 
  Porites, 
  0*62 
  and 
  

   95*94; 
  for 
  an 
  Oculina, 
  0*36 
  and 
  96*20; 
  for 
  Millepora 
  alcicor?iis, 
  

   0-41 
  and 
  95*86; 
  for 
  another 
  Millepora, 
  0*77 
  and 
  94*39; 
  for 
  the 
  

   reef 
  rock 
  of 
  the 
  Bermudas, 
  1*64 
  and 
  95*43; 
  for 
  10 
  species 
  of 
  

   shells 
  of 
  Gastropods 
  and 
  Lamellibranchs 
  from 
  the 
  outer 
  edge 
  

   of 
  the 
  reef, 
  0*19 
  and 
  97*32; 
  but 
  for 
  a 
  fine 
  mud 
  of 
  terra 
  cotta 
  

   color 
  from 
  the 
  lagoon, 
  4*04 
  and 
  92*93. 
  

  

  The 
  author 
  concludes 
  that 
  the 
  magnesia 
  of 
  dolomite 
  has 
  prob- 
  

   ably 
  come 
  from 
  an 
  organic 
  source 
  ; 
  and 
  that 
  consequently 
  the 
  

   hypothesis 
  of 
  Dana, 
  which 
  derives 
  it 
  from 
  the 
  sea- 
  water 
  of 
  

   evaporating 
  basins, 
  appears 
  to 
  be 
  unnecessary. 
  

  

  We 
  add 
  here 
  some 
  additional 
  facts. 
  

  

  Damour, 
  in 
  1850, 
  published 
  an 
  analysis 
  (Bull. 
  G. 
  S. 
  de 
  France, 
  

   vii, 
  675) 
  of 
  a 
  specimen 
  from 
  Brehat, 
  on 
  the 
  coast 
  of 
  northwestern 
  

   France, 
  which 
  he 
  called 
  a 
  Millepora 
  (M. 
  cervicornis), 
  but 
  also 
  

   said 
  it 
  was 
  one 
  of 
  the 
  Algce, 
  and 
  of 
  concretion-like 
  form. 
  It 
  

   afforded 
  him 
  8*61 
  per 
  cent 
  of 
  magnesium 
  carbonate 
  and 
  87*32 
  of 
  

   calcium 
  carbonate. 
  

  

  