Subdivisions of the Paleozoic Strata of Great Britain. 351 
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 palzontologically, 
well separated from the Silurian by the intervention of the Ma 
Hill sandstone. Commencing with the May Hill group there is 
asudden 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 sndden 
(and almost complete) suppression 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 
demargation is, I think, perfectly set at rest: nor does there ap- 
pear 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 Wen- 
lock) sandstone to the upper Ludlow rocks inclusive, are by no 
means so well separated from the old red sandstone (or Devonian 
tocks) of Herefordshire and South Wales. The old red sand- 
stone was subdivided by Buckland and Conybeare into the Tile- 
Stone group, the Cornstene group, and the red sandstone and 
conglomerate group; and this triple division, with many new 
and important additional 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. 
‘he Tilestone was struck off from the Devonian series, and 
. 
placed at the top of the Silurian. There is, therefore, in the 
fishes begin first to appear in the Upper Silurian groups, and be- 
come eminently characteristic of the Devonian, we seem (through 
British rocks; and assuredly the classification of British rocks 
should, in the first instance, be based on British evidence. But 
may remark, by the way, that the lower Devonian groups of 
