﻿1864.] 
  WINTLE 
  HOBART. 
  465 
  

  

  took 
  the 
  place 
  of 
  the 
  moving 
  stone, 
  its 
  grinding 
  power 
  vastly 
  increased, 
  

   and 
  in 
  great 
  part 
  due 
  to 
  the 
  moving 
  glacier 
  above 
  it. 
  

  

  The 
  rock-basins 
  of 
  Nova 
  Scotia 
  are 
  shallower 
  than 
  those 
  of 
  Italy, 
  

   because 
  in 
  the 
  one 
  case 
  the 
  rocks 
  operated 
  upon 
  are 
  harder, 
  being 
  

   metamorphosed 
  schists, 
  than 
  those 
  in 
  the 
  other 
  case. 
  

  

  Many 
  of 
  the 
  lakes, 
  especially 
  the 
  smaller, 
  have 
  been 
  obliterated 
  

   since 
  the 
  glacial 
  epoch 
  by 
  the 
  formation 
  of 
  a 
  freer 
  passage 
  for 
  the 
  

   waters, 
  or 
  by 
  the 
  growth 
  of 
  vegetation, 
  or 
  by 
  sedimentary 
  deposits. 
  

   But 
  in 
  North 
  America 
  and 
  North 
  Europe 
  they 
  are 
  still 
  very 
  nume- 
  

   rous 
  : 
  the 
  preservation 
  of 
  these 
  lakes 
  is 
  accounted 
  for 
  by 
  the 
  length 
  

   and 
  severity 
  of 
  their 
  winters. 
  The 
  preservative 
  action 
  of 
  the 
  ice 
  is 
  

   twofold 
  : 
  it 
  encloses 
  and 
  carries 
  off 
  mud, 
  gravel, 
  and 
  stones 
  from 
  the 
  

   beaches, 
  and 
  deposits 
  the 
  same 
  towards 
  the 
  outlets 
  of 
  the 
  lakes, 
  and 
  

   thus 
  the 
  accumulation 
  of 
  material 
  there 
  counteracts 
  the 
  erosive 
  

   action 
  of 
  the 
  streams 
  issuing 
  through 
  these 
  outlets. 
  It 
  is 
  therefore 
  

   directly 
  antagonistic 
  to 
  the 
  obliterating 
  power 
  of 
  running 
  water, 
  

   and 
  it 
  is 
  even 
  probable 
  that 
  some 
  of 
  the 
  lakes 
  may 
  be 
  enlarged 
  and 
  

   deepened 
  by 
  the 
  tearing 
  away 
  of 
  their 
  banks 
  and 
  the 
  raising 
  of 
  their 
  

   outlets. 
  

  

  8. 
  A 
  Sketch 
  of 
  the 
  Principal 
  Geological 
  Features 
  of 
  Hobart, 
  

  

  Tasmania. 
  By 
  S. 
  H. 
  Wintle, 
  Esq. 
  

  

  [Communicated 
  by 
  Sir 
  Roderick 
  I. 
  Murchison, 
  K.C.B., 
  F.R.S., 
  F.Gr.S.] 
  

  

  [Abridged.] 
  

  

  1. 
  Trap-rocks. 
  — 
  The 
  hills 
  upon 
  which 
  Hobart 
  is 
  built*, 
  as 
  well 
  

   as 
  those 
  for 
  the 
  most 
  part 
  immediately 
  surrounding 
  it, 
  are 
  composed 
  

   of 
  New 
  Bed 
  Sandstone, 
  capped, 
  in 
  nearly 
  every 
  instance, 
  with 
  trap 
  

   or 
  greenstone, 
  which 
  varies 
  considerably 
  in 
  mineral 
  composition, 
  

   from 
  a 
  felspathic 
  subcrystalline 
  rock 
  to 
  a 
  compact 
  hornblendic 
  green- 
  

   stone 
  which 
  is 
  used 
  for 
  metalling 
  the 
  roads 
  f. 
  This 
  trap 
  seldom 
  

   exhibits 
  that 
  step-like 
  appearance 
  said 
  to 
  be 
  characteristic 
  of 
  the 
  

   English 
  trappean 
  rocks 
  ; 
  but 
  this 
  may 
  be 
  ascribed 
  to 
  two 
  causes, 
  

   namely, 
  its 
  invariably 
  exhibiting 
  numerous 
  joints, 
  which 
  cross 
  each 
  

   other 
  at 
  different 
  angles, 
  and 
  also 
  to 
  its 
  having 
  been 
  shattered 
  by 
  

   eruptive 
  forces 
  at 
  some 
  subsequent 
  period. 
  The 
  thickness 
  of 
  this 
  

   trap 
  in 
  certain 
  localities 
  is 
  very 
  considerable. 
  A 
  quarry 
  near 
  the 
  

   High 
  School, 
  in 
  this 
  rock, 
  is 
  at 
  the 
  present 
  time 
  upwards 
  of 
  100 
  

   feet 
  in 
  depth, 
  and, 
  from 
  the 
  dip 
  and 
  strike 
  of 
  the 
  underlying 
  sand- 
  

   stone, 
  I 
  estimate 
  that 
  it 
  will 
  attain 
  a 
  depth 
  of 
  200 
  feet 
  more 
  before 
  

   the 
  sandstone 
  is 
  reached. 
  

  

  From 
  Professor 
  Selwyn's 
  Report 
  on 
  the 
  Coal-measures 
  of 
  this 
  part 
  

   of 
  the 
  island, 
  it 
  would 
  seem 
  that 
  a 
  capping 
  of 
  trap 
  is 
  the 
  prevailing 
  

   feature 
  of 
  sandstone-hills 
  throughout 
  the 
  district. 
  At 
  Knocklofly 
  

  

  * 
  These 
  hills 
  are 
  as 
  follows 
  : 
  — 
  

  

  Knocklofty 
  1700 
  feet. 
  

  

  Proctors 
  Hill 
  1100 
  „ 
  

  

  Mount 
  Nelson 
  1200 
  „ 
  

  

  Surveyors 
  Hill 
  1080 
  „ 
  

  

  t 
  See 
  specimen 
  No. 
  2, 
  in 
  Coll. 
  Greol. 
  Soc. 
  

  

  