HICKS MENEYIA.N FOSSILS. 



181 



next ; for Protosjjongki fen?stra!a occurs in the Longmyud group, in 

 the Meneviau group, and again in the Upper Lingula flags, to the 

 base of the Tremadoc rocks, where it was found by me a few years 

 since when examining these rocks in IS'orth Wales. These therefore 

 continued to live on during the deposition of from 8000 to 10,000 feet 

 of strata. None of the other Menevian species have any considerable 

 range ; and but very few of the genera pass beyond the Menevian boun- 

 dary. Out of ten genera of Trilobites, one only, the little Agnostus, 

 passes upwards*, and not one species oat of the whole number, in all 

 twenty-nine. Indeed the range of all these species is very limited ; 

 and each one seems to mark a special zone, where it flourished for a 

 time and then disappeared, perhaps to be followed by others of 

 the same type, but never to reappear. The Crustacea are therefore, 

 in these earliest rocks, the surest indices of the age of the strata, 

 and the best guides in defining the several zones ; for the more per- 

 fect the forms and the higher the development, the less likely are 

 they to have a great range. 



It seems difficult to conceive that distinct and separate creations 

 should so frequently take place ; and yet how strange that, up to the 

 present time, no satisfactory cause has been assigned for this con- 

 stant dying out of old forms and replacement by new ones ! It seems 

 possible to conceive that physical conditions should occasionally 

 produce sudden destruction or disappearance, for a time, of particular 

 forms, and a recurrence with a reappearance of the same condi- 

 tions—as for instance when a sudden elevation of a portion of the 

 sea-bottom took place, forming a beach or shallow water, and then 

 became again depressed. But when we have an even deposit, 

 with some of the forms continuing while others are dying out and 

 new ones coming into the field, we must look for other than 

 physical conditions, and for some cause hitherto unexplained. 

 Evidences of frequent oscillations of the sea-bottom during the 

 deposition of the Cambrian rocks are of common occurrence in 

 North and South Wales. The results obtained by the examination 

 of the successive strata in the neighbourhood of St. David's may 

 be briefly summed up thusf : — 



1. A pre-Cambrian island, composed of quartziferous beds inter- 

 stratified with dark green sandstones, and with a strike from N.W. to 

 S.E., and hence discordant with that of the overlying Cambrian strata. 

 This is possibly one only of a group which existed at this period in 

 this neighbourhood ; for another occurs, under almost the same con- 

 ditions, about eight miles from St. David's, and surrounded by beds 

 much like those at St. David's. 



2. Fine-grained quartziferous hornstones of a bright green colour, 

 with a strike from N.E. to S.W., and resting on the sides of, but not 

 overlying, the pre-Cambrian island. During the deposition of these 



* Conocoryphe also is mentioned ; but I agree with Mr. Belt in thinking 

 that the so-called Conocoryphe of the upper rocks is generically distinct from 

 the Menevian ones. 



t I have already given the thickne.ss of cacli division at p. 301, Quart. Journ, 

 Geol. Soc. vol. xxvii. 



VOL. xxviii. — rAET I. 



