﻿166 
  

  

  J. 
  8. 
  Diller 
  — 
  Crater 
  Lake, 
  Oregon. 
  

  

  tral 
  volcano. 
  The 
  crest 
  of 
  the 
  rim 
  varies 
  in 
  height 
  from 
  6759 
  

   to 
  8228 
  feet 
  above 
  the 
  sea, 
  i. 
  e. 
  520 
  to 
  1989 
  above 
  the 
  lake. 
  

   Its 
  prominences 
  stand 
  at 
  the 
  head 
  of 
  spurs 
  radiating 
  from 
  the 
  

   lake. 
  Some 
  of 
  these 
  ridges 
  were 
  formed 
  by 
  single 
  streams 
  of 
  

   lava, 
  but 
  others 
  result 
  from 
  erosion 
  and 
  are 
  separated 
  by 
  deep 
  

   canyons, 
  such 
  as 
  those 
  of 
  Sand 
  Creek 
  and 
  Sun 
  Creek, 
  which 
  

   pass 
  directly 
  through 
  the 
  rim. 
  

  

  The 
  rim 
  is 
  composed 
  wholly 
  of 
  lava 
  streams 
  and 
  beds 
  of 
  

   volcanic 
  conglomerate, 
  dipping 
  away 
  from 
  the 
  lake 
  as 
  shown 
  

   by 
  the 
  accompanying 
  figure. 
  This 
  is 
  the 
  normal 
  composition 
  

   and 
  structure 
  of 
  the 
  basal 
  portion 
  of 
  a 
  large 
  volcano. 
  

  

  Mt 
  Mazama 
  

  

  Fig. 
  1. 
  — 
  Section 
  of 
  Crater 
  Lake 
  and 
  its 
  Rim 
  with 
  the 
  probable 
  outline 
  of 
  Mt. 
  

   Mazama, 
  structural 
  details 
  generalized 
  — 
  vertical 
  and 
  horizontal 
  scales 
  the 
  same. 
  

  

  The 
  southern 
  and 
  western 
  parts 
  of 
  the 
  rim 
  are 
  made 
  up 
  of 
  

   many 
  beds 
  ; 
  those 
  of 
  lava 
  generally 
  predominate, 
  both 
  in 
  size 
  

   and 
  number. 
  The 
  northeastern 
  portion 
  of 
  the 
  rim 
  at 
  the 
  Pali- 
  

   sades 
  is 
  made 
  up 
  almost 
  wholly 
  of 
  one 
  great 
  flow. 
  The 
  same 
  

   is 
  true 
  to 
  a 
  less 
  extent 
  at 
  Llao 
  Rock, 
  which 
  is 
  formed 
  of 
  a 
  short 
  

   but 
  broad 
  stream 
  over 
  1200 
  feet 
  in 
  thickness. 
  The 
  exposures 
  

   upon 
  the 
  inner 
  slope 
  of 
  the 
  rim 
  show 
  sections 
  of 
  the 
  lava 
  

   streams 
  radiating 
  from 
  the 
  lake, 
  and 
  that 
  of 
  Llao 
  Rock 
  furnishes 
  

   a 
  good 
  example. 
  Its 
  greatest 
  thickness 
  is 
  in 
  the 
  middle, 
  where 
  

   it 
  fills 
  an 
  ancient 
  valley, 
  down 
  the 
  northwestern 
  slope 
  of 
  the 
  

   rim 
  and 
  tapers 
  upon 
  both 
  sides 
  to 
  a 
  thin 
  edge 
  as 
  seen 
  in 
  Plate 
  

   Y. 
  This 
  massive 
  flow 
  rests 
  locally 
  on 
  an 
  irregular 
  layer 
  of 
  

   pumice 
  and 
  is 
  overlain 
  by 
  the 
  same 
  sort 
  of 
  material. 
  

  

  Andesites 
  predominate, 
  especially 
  among 
  the 
  earlier 
  lava 
  

   flows 
  so 
  well 
  exposed 
  in 
  section 
  upon 
  the 
  inner 
  slope 
  of 
  the 
  

   rim 
  ; 
  but 
  rhyolites 
  are 
  common 
  among 
  the 
  later 
  ones, 
  and 
  are 
  

   usually 
  associated 
  with 
  pumice. 
  Basalts 
  were 
  not 
  observed 
  in 
  

   the 
  rim. 
  However, 
  they 
  occur 
  upon 
  its 
  outer 
  slope 
  several 
  

   miles 
  away 
  from 
  the 
  crest. 
  They 
  are 
  connected 
  with 
  promi- 
  

   nent 
  cinder 
  cones 
  adnate 
  to 
  the 
  once 
  greater 
  central 
  peak 
  and 
  

   are 
  the 
  newest 
  lavas 
  of 
  the 
  region, 
  excepting 
  that 
  of 
  Wizard 
  

   Island 
  within 
  the 
  lake. 
  This 
  succession 
  of 
  lavas 
  has 
  been 
  

  

  