the ancient kocks of st. davlt>'s. 181 



Genekal Conclusions. 



In the preceding remarks I have endeavoured, in addition to giving 

 a minute description of the Arenig and Llandeilo groups, and their 

 relation to the groups immediately above and below, to show that 

 the sequence of the rocks of which these form a part, in the pro- 

 montory of St. David's is a perfectly continuous one from the base 

 of the Longmynd or Harlech group to the Llandeilo group, and that 

 in this section there are no lines of division stronger than what 

 would naturally occur in a great series deposited over a sea-bottom 

 becoming gradually depressed, and subject to the ordinary physical 

 influences which must have prevailed during such a change. 



The paleeontological record made out in these ancient rocks is 

 shown to be more complete than has hitherto been found to be the 

 case in any other single section ; and consequently it has enabled 

 us to recognize the proper position of groups hitherto but imperfectly 

 defined, and to correct the limits which had been incorrectly given. 



In instituting comparisons between the succession as here made 

 out with that found in other districts, the evidence has in each case 

 been most carefully considered, and no facts have been accepted 

 which have not been critically examined. Some may object, how- 

 ever, to a comparison of sections in districts even so far apart as 

 North and South Wales, on the ground that the deposits might possibly 

 have been formed under somewhat different conditions ; but these 

 objections, if raised, must give way when it is considered that the 

 Welsh area comprised only a very small proportion of the oceanic area 

 which prevailed when these series were deposited, as indicated by the 

 occurrence in Sweden, Belgium, France, Bohemia, and Spain on the 

 one hand, and in North America on the other, of rocks with the 

 same order of succession in the organic remains, and the same 

 general sequence of beds, subject only to what might be called local 

 differences, as of thickness &c. 



There are evidences also present within small areas, such as the 

 presence of contemporaneous lavas and ashes and of calcareous beds 

 at certain stages in the strata, which enable us frequently, even 

 without the paleeontological evidence, to recognize readily the position 

 of certain beds. The series may, however, on the other hand vary 

 considerably in thickness even within a small area ; and sometimes 

 the character of the deposits may change abruptly, as in the case of 

 the great Grit-bed in North Wales, and the Stiper Stones of Shrop- 

 shire, which were evidently portions of a sandbank extending 

 over those areas in a deep sea in which elsewhere, even within 

 narrow limits, fine muddy deposits were being thrown down. But, 

 taking the series collectively, these local differences are not of much 

 importance ; and they do not usually interfere with the order of the 

 general succession so as to prevent its being easily followed out. 



In the section at St. David's the strongest palseontological breaks 

 occur at the close of the Menevian group and at the close of the 

 Tremadoc group ; and it is there I consider the chief lines for the 

 division of the series should be placed. The Menevian group and 



o2 



