File:The Quarterly journal of the Geological Society of London (13937286652).jpg

470

 ME. 
 S. 
 S. 
 BfCIOIAX 
 OX 
 THE 
 

Considering

 the 
 very 
 great 
 development 
 of 
 the 
 Toarcian 
 limestone 
 

in

 the 
 Cotteswolds 
 it 
 may 
 be 
 advantageous 
 to 
 give 
 a 
 comparison 
 of 
 

the

 strata 
 of 
 that 
 district 
 with 
 a 
 section 
 of 
 the 
 strata 
 in 
 Dorset 
 (fig. 
 2, 
 

facing

 p. 
 456). 
 This 
 correlation 
 is 
 entirely 
 founded 
 on 
 the 
 position 
 of 
 

the

 different 
 species 
 of 
 Ammonites 
 ; 
 and 
 for 
 further 
 information 
 on 
 

this

 matter 
 the 
 reader 
 is 
 referred 
 to 
 my 
 Ponograph 
 on 
 " 
 Inferior- 
 

Oolite

 Ammonites," 
 Palaeontographical 
 Society, 
 p. 
 91. 
 

The

 horizontal 
 diagram 
 (fig. 
 3, 
 p. 
 468) 
 is 
 intended 
 to 
 represent 
 the 
 

Inferior-Oolite

 strata 
 north 
 and 
 south 
 of 
 the 
 Mendips, 
 with 
 their 
 

geographical

 extension 
 according 
 to 
 the 
 correlation 
 just 
 given, 
 on 
 a 
 

line

 from 
 Leckhampton 
 Hill 
 to 
 .Burton 
 Bradstock. 
 Apparently 
 the 
 

Mendip

 range 
 must 
 have 
 acted 
 as 
 a 
 barrier 
 between 
 the 
 areas 
 on 
 

each

 side 
 of 
 it 
 : 
 but 
 it 
 is 
 more 
 curious 
 to 
 find 
 that 
 a 
 change 
 in 
 litho- 
 

logy

 in 
 the 
 Sodbury 
 district, 
 a 
 change 
 which 
 I 
 have 
 demonstrated 
 in 
 

the

 case 
 of 
 the 
 sands, 
 should 
 coincide 
 with 
 an 
 outcrop 
 of 
 Carboniferous 
 

Limestone.

 This 
 is 
 part 
 of 
 a 
 patch 
 which 
 extends 
 to 
 Charfield 
 and 
 

reappears

 across 
 the 
 Severn 
 and 
 no 
 doubt 
 formed, 
 at 
 that 
 period, 
 a 
 

subsidiary

 range 
 of 
 hills 
 joining 
 the 
 Mendip 
 axis 
 at 
 right 
 angles. 
 

However,

 the 
 character 
 which 
 this 
 figure 
 brings 
 into 
 greatest 
 

prominence

 is 
 the 
 remarkable 
 persistence 
 of 
 a 
 hiatus 
 or 
 lacuna, 
 

due

 to 
 the 
 absence 
 of 
 certain 
 strata, 
 in 
 the 
 middle 
 of 
 what 
 is 
 called 
 

Inferior

 Oolite. 
 This 
 is 
 the 
 same 
 hiatus 
 which 
 Eugene 
 Deslongs- 
 

champs

 finds 
 in 
 Xormandy, 
 and 
 which 
 he 
 proposes 
 shall 
 mark 
 the 
 

uppermost

 limit 
 of 
 his 
 Infra-Oolitic 
 marls 
 ; 
 this 
 is 
 the 
 same 
 hiatus 
 

which

 Dr. 
 Yacek 
 says 
 occurs 
 over 
 the 
 continent 
 generally, 
 and 
 

which

 he 
 proposes 
 as 
 the 
 uppermost 
 limit 
 of 
 the 
 Lias 
 ; 
 this 
 is 
 the 
 

hiatus

 which 
 I, 
 following 
 d'Orbigny, 
 propose 
 to 
 take 
 as 
 the 
 dividing 
 

line

 between 
 the 
 " 
 Toarcien 
 " 
 and 
 the 
 "Bajocien" 
 ; 
 this 
 is 
 the 
 hiatus 
 

which

 is 
 accompanied 
 palaeontologically 
 by 
 the 
 sudden 
 exit 
 of 
 nearly 
 

all

 the 
 Hildoceratidae, 
 or 
 ' 
 true 
 Falciferi." 
 

One

 last 
 subject 
 remains 
 to 
 be 
 dealt 
 with, 
 namely, 
 the 
 matter 
 of 
 

mapping.

 In 
 connexion 
 herewith 
 the 
 proposed 
 classification 
 would 
 

introduce

 some 
 advantages, 
 and 
 would 
 certainly 
 get 
 rid 
 of 
 such 
 

anomalies

 as 
 the 
 same 
 zone 
 being 
 mapped 
 as 
 " 
 Mid 
 .ford 
 Sands" 
 

in

 one 
 county 
 and 
 as 
 Upper 
 Lias 
 Clay 
 in 
 another 
 : 
 or, 
 again, 
 as 
 

"

 Midford 
 Sands" 
 at 
 one 
 place 
 and 
 Inferior-Oolite 
 Limestone 
 some- 
 

where

 else. 
 It 
 would 
 probably 
 be 
 best 
 for 
 this 
 purpose 
 to 
 divide 
 

the

 Toarcian 
 into 
 Upper 
 and 
 Lower, 
 although 
 there 
 is 
 between 
 

them,

 lithologically, 
 no 
 marked 
 break 
 at 
 any 
 point, 
 and, 
 palaeonto- 
 

logically,

 only 
 a 
 small 
 one. 
 This 
 palaeontological 
 break 
 occurs 
 at 
 

the

 end 
 of 
 the 
 Opalinum-zone 
 with 
 the 
 extinction 
 of 
 the 
 genus 
 

Grammoceras

 — 
 only 
 one 
 species 
 of 
 which, 
 I 
 believe, 
 survives 
 into 
 the 
 

Murchisonce-zone.

 Therefore, 
 the 
 Falciferum-, 
 Commune-, 
 Jnrense-, 
 

and

 Opalinum-zon.es 
 form 
 the 
 Lower 
 Toarcian. 
 They 
 would 
 

require

 the 
 following 
 changes 
 among 
 others, 
 namely 
 the 
 abolition 
 of 
 

the

 dividing-line 
 now 
 drawn 
 between 
 Upper 
 Lias 
 * 
 and 
 " 
 Midford 
 

 Throughout 
 a 
 large 
 part 
 of 
 Dorset 
 and 
 Somerset 
 the 
 Upper 
 Lias 
 Clay 
 is 
 

not

 mapped, 
 although 
 to 
 a 
 great 
 extent 
 it 
 overlies 
 what 
 is 
 marked 
 as 
 "^2, 
 

Middle

 Lias." 
 For 
 instance, 
 at 
 South 
 Pethertom 
 it 
 is 
 exposed 
 to 
 a 
 depth 
 of 
 

nearly

 seven 
 feet 
 on 
 the 
 top 
 of 
 the 
 Marlstone 
 ; 
 but 
 nothing 
 is 
 said 
 about 
 it 
 on 
 

the