1852.] SEDGWICK ON THE LOWER PALiEOZOIC ROCKS. 145 



wrangling about the use or abuse of tbe word " system," I will pro- 

 visionally separate the whole lower palcBozoic series into two great 

 natural subdivisions — Cambrian and Silurian ; each of which may be 

 again subdivided into a series of stages or groups, which, collectively, 

 I here designate by the names Cambrian Series and Silurian 

 Series. The Cambrian and Silurian collective groups, thus defined, 

 have a well-marked physical separation ; and the Silurian groups are, 

 not unusually, unconformable to the Cambrian : and although several 

 fossils are common to the two collective groups, especially near the 

 planes of junction, yet the fossils of the well-defined lower stages of 

 the Cambrian series are very widely distinct from the fossils of the 

 upper stages of the Silurian series. I believe that this is the case in 

 Wales and Siluria ; and I am certain that it is the case in the Cum- 

 brian cluster of mountains. The fossils of the Coniston calcareous 

 slates hardly reappear at all, and certainly not as a group, among the 

 very numerous fossils of the rocks between Kendal and Kirkby Lons- 

 dale (Upper Ludlow). Hence, on mere palseontological grounds, it 

 would produce nothing but confusion were we to designate the Lud- 

 low rocks south of Kendal, and the calcareous slates of Coniston, &c. 

 as one system, while we adopt the restricted use of the word "system "' 

 now in common use. 



After these preliminary remarks, I proceed to enumerate the 

 several groups into which the whole Cambrian and Silurian series may, 

 I think, be conveniently separated. I profess not to describe the 

 granite and other igneous and unstratified rocks ; but I may just 

 notice the metamorphic slates of Anglesea and Caernarvonshire, com- 

 posed of quartz-rock, quartzose mica-slate, quartzose chloritic slate^ 

 crystalline limestone, serpentine, &c. That they are of great anti- 

 quity is certain ; for they appear to underlie, and they certainly do 

 not overlie, the old rocks in the great S.W. promontory of Caernar- 

 vonshire. That they are truly hjrpozoic, or that they are older than 

 any of the unaltered slate-groups of the Principality, is by no means 

 certain ; but these are points quite foreign to the discussions of this 

 paper. 



On the eastern side of the Menai Straits is an expansion of some 

 dark slates, which I was at one time induced to consider (hypothetic- 

 ally, however, and without any direct proof) as the lowest unaltered 

 slates in North Wales, and perhaps the equivalents of the Skidd aw 

 slate. In 1846 I changed this view, chiefly on mineral evidence, and 

 arranged the dark Menai slates in the same group with the black 

 slates of Tremadoc. The change, which I made on imperfect evi-- 

 dence, has been since established on better evidence by Professor 

 Ramsay and the gentlemen of the Government Survey. 



The whole Cambrian series is exhibited, in vast undulations, from 

 the Menai to the Berwyns ; and a part of it, again, in a system of 

 what might be called short independent waves, on the east side of the 

 Berwyns, until the last beds of the series become buried under the 

 carboniferous limestone. But, if we extend our views to the north 

 end of the great undulating series, we find (not, however, without con- 

 tinual breaks and dislocations) the prevailing strike and dip so 



VOL. viii. — part i. I4 



