404 Dr. H. UicIiS — Life Zones in PalcBO%oic Rochs. 



fauna extends through a great thickness of strata, as I shall be able 

 to show when I refer more particularly to the Middle Cambrian. 

 Almost immediately after Mi-. Walcott's announcement was made, 

 Prof. C. Lapworth published a highly interesting account of the 

 discovery by him in Shropshire of the OleneUus fauna, and he there 

 states that " the presence of traces of the OleneUus fauna in the 

 West of England " was known to him for some time, the first 

 recognisable fragments having been detected by him " on the 

 flanks of Caer Caradoc in 1885, but they were too imperfect for 

 description." Better material had by this time been obtained, and 

 he was now able to give the provisional name of OleneUus Callavei 

 to one of the forms, and to add several other fossils known to be 

 characteristic of the OleneUus zone. He further says ^ that "this 

 Lower Cambrian or OleneUus formation of the Shropshire area 

 consists of two main members, viz. the basal quartzite of Lawrence 

 Hill and Caer Caradoc, and an overlying green sandstone, the 

 Coraley Sandstone (Hollybush Sandstone of Dr. Callaway). This 

 formation follows unconformably upon the so-called Uriconian rocks 

 of the district, aud occurs in many localities, as at Lilleshall, the 

 Wrekin, Caer Caradoc, Cardington, etc." 



In the Geological Magazine for December, 1891, Prof. Lapworth 

 published a full description of OleneUus Callavei, with figures ; and 

 stated that in Shropshire, as elsewhere, " we find the Cambrian 

 divisible into three sections — an Upper Cambrian above, marked by 

 the presence of the genus Olenus (Olenidian) ; a Middle Cambrian 

 group with Faradoxides (Menevian or Paradoxidian) ; and finally a 

 Lower Cambrian {OleneUus zone) or basal group (possiblj'^ of some- 

 what diiierent systematic importance), distinguished by the presence 

 "of OleneUus." At the meeting of the British Association in 1891, 

 Sir A. Geikie announced the discovery of OleneUus by the Geological 

 Surveyors in the North-West of Scotland, and since then additional 

 species of that genus have been obtained by them as well as some 

 of the usually associated organisms. This important discovery of 

 typical Lower Cambrian fossils overlying Torridon Sandstone 

 proved that the latter must be of higher antiquity, hence of pre- 

 Cambrian age. Up to the present time the usually overlying 

 Faradoxides and Olenus zones have not been found in Scotland, 

 It seems pretty evident that the Cambrian rocks must be com- 

 paratively thin in the North- West of Scotland ; in this respect 

 simulating the succession in Scandinavia rather than that in Wales. 

 There can be no doubt that the genera OleneUus, Faradoxides, and 

 Olenus in the areas where they have been found to succeed each 

 other in conformable sediments do mark very definite periods in the 

 world's history; but such arbitrary lines are not natural, and we 

 must expect to find from time to time that the limit assigned to 

 a genus will have to be extended as new areas are being explored. 

 Where a genus, which has been sufficiently abundant to characterize 

 a main zone, disappears suddenly, there is usually some indication 

 in the deposits of at least a slight physical change. At St. David's 

 1 Geol. Mag. Dec. III. Vol. V. p. 485, and "Natui-e," vol. xxis. p. 213. 



