INTRODUCTION. XXVil 
General Conclusions. 
On casting an eye over the above short Synopsis, or Tabular View, of the whole Pale- 
ozoic series, several conclusions seem to force themselves on any well-informed reader: and 
I will enumerate them here, though they involve a partial repetition of what has been stated 
above. A more detailed discussion of them will, I hope, before long form the subject of 
another Essay. 
First. There is a frequent difficulty in separating the collective groups, such as the 
Cambrian, Silurian, Devonian, &c., by well-defined lines of demarcation: and this difficulty is 
very little affected by our nomenclature; for it obviously remains the same, by whatever names 
we describe these collective groups. But the Cambrian Series is now, physically and palzeonto- 
logically, well separated from the Silurian by the intervention of the May Hill Sandstone. 
Commencing with the May Hill group there is a sudden change of mineral type, and an 
obvious physical break, sometimes marked by a change of strike and clear discordancy of 
position ; and along with these changes there is also a sudden (and almost complete) sup- 
pression of the most abundant and characteristic of the older organic types. So far as 
regards the evidence supplied by Wales, and the bordering English counties, and by the north 
of England, the question respecting this line of demarcation is, I think, perfectly set at rest: 
nor does there appear to be anything in the development of the older Paleozoic series of the 
continents of Europe or America which is opposed to this conclusion. 
Secondly. The Silurian groups, from the May Hill (or Wenlock) sandstone to the upper 
Ludlow rocks inclusive, are by no means so well separated from the Old Red sandstone (or 
Devonian rocks) of Herefordshire and South Wales. The Old Red sandstone was subdivided 
by Buckland and Conybeare into the Tilestone group, the Cornstone group, and the Red Sand- 
stone and conglomerate group; and this triple division, with many new and important addi- 
tional details, was adopted in the “Silurian System.” Chiefly on the combined physical and 
fossil evidence supplied by the corresponding groups of Westmoreland, this triple division of 
the Old Red sandstone was afterwards abandoned. The Tilestone was struck off from the 
Devonian series, and placed at the top of the Silurian. There is, therefore, in the typical 
Silurian country such a graduation or passage between the Silurian and Devonian groups as 
to place a physical difficulty in our way when we attempt to draw a line between them. 
This line, as it is drawn at present, might, however, be considered as very well fixed and 
determined, were it not for the enormous development of the Old Red sandstone of Scot- 
land, which (as before hinted) seems to descend below the base line of the Devonian series of 
Siluria. And when we bear in mind that Fishes begin first to appear in the Upper Silurian 
groups, and become eminently characteristic of the Devonian, we seem (through this Verte- 
brate Class of the Animal Kingdom) to make out a nearer connexion between the Silurian 
and Devonian groups that was at one time imagined. 
I am here discussing no evidence but what is supplied by British rocks; and assuredly 
the classification of British rocks should, in the first instance, be based on British evidence. 
But I may remark, by the way, that the lower Devonian groups of the continent (e. g. those 
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