Geohfj'ical Society. 495 



tlie unik-rlying hlue shali^s, iiltli>)iit^li the irrea;ulirity of the eroded 

 surface does not ai)])ear to be great in tlie somewhat poor exjwsures. 

 The beds liave been examined in a series of trenches situated 

 near the Abbey Mound in Martshill Hayes, and have yielded over 

 fifty ditferent species of trilobites — each ranging through one or 

 more of about fifteen fossiliferous horizons in the sequence. The 

 fauna sliows marked afUnities with those of the equivalent beds in 

 Wales, Scandinavia, and Bohemia, and the following subdivision 

 into zones has been attempted : — 



Horizon. 



G3-G1 Upper P. -(iartdis Fauna. 



F3-F1 Lower P.-rfai'idi8 Fauna. 



E 3-E 1 (pars) Hartsh Mia Fauna. 



E 1 (pars) -D 1 Upper h icksii Fauna. 



C 3-B 1 Lower /u'cfcjtu Fauna. 



A4-A1 P.-awrora Fauna. 



The iniportance of the Abbey Shales lies in the possibility which 

 they afford of a close correlation between the Swedish and British 

 Paradoxidian faunas, for the intermediate geographical position is 

 accompanied by an intermediate type of fauna forming a link 

 between these two well-known areas. 



The following is a brief summary of the correlation that 

 appears to agree best with the available evidence: — ■ 



South Wales. Haetshill. Scandinavia. 



f Upper P.-daiidis Fauna. P.-davidin Zone. 



„ , .,• ry i Lower P.-c?((firfw Faima. P. -dacidis Zone. 



P.-daruU)! Zone. ■{ 



[^Hartishillia-inJlntnFeimifi,. C.-.-egfirrhx Sub-zone. "^ 



r, ..... „ ( Upper hicliisii Fauna. | I • 



P.-hick.iii Zone. J '^ yAynostits-parvifroiu Sub-zone. ! g' ? 



I Iiower hicksii Fauna. 1 ^ ^ 



S 2. 



p. -aurora Zone, P.-aurora Fauna. Ct.-exsidans Sub-zone (pars). J 



In addition, the evidence suggests that the Upper Purley Shales 

 correspond to the rest of the C't.-exsuhins Zone and also to the 

 P.-dlanJiciis Zones of Sweden. 



The Abbey Shales appear to have been deposited in rather a 

 sliallow open sea, in which slight changes in conditions inti'oduced 

 marked variations in the rock-types. Corresponding to these 

 alternating physical conditions there occur alternations of faunas — 

 a good example being afforded by Agnostits /Ya", found in the coarse 

 shales, and A. cf. intermedins, found in the blue shale. 



Towards the close of the Middle Cambrian Period excessive 

 shallowing of the water introduced a period of marine erosion — 

 causing a break in the sedimentary sequence with the elimination 

 of the equivalents of the P .-forchhn mmeri Zone, and probably a 

 portion of the P.-davidis Zone. This break appears to have ex- 

 tended over a large part of Great Britain, attention being di-awn 

 to the fact that none of the characteristic forms which occur 

 exclusively in the rich P.-forchhammeri Zone of Scandinavia have 

 been noted fnun anv localitv in Great Britain. 



