238 



THE CAMBRIAN. 



[HULL. 81. 



is thence expanded through a considerable portion of south Wales. In one part of 

 its course it is based on beds oft limestone and calcareous slate, but on the whole it 

 contains Less calcareous matter than the Silurian system and has fewer organic 

 remains. Beds of good roofing slate occur, and a perfect slaty cleavage is often 

 observed in it transverse to the stratification, but other parts of it are of a coarse 

 mechanical texture. To the next inferior group he gave the name of Middle Cam- 

 brian. It composes all the higher mountains of Caernarvonshire and Merionethshire, 

 and abounds in tine roofing slate, alternating with and apparently passing into ir- 

 regularly interstratiried masses of porphyry. Some portions of it are coarse and me- 

 ehanical, and it contains (for example at the top of Snowdon) a few organic remains 

 and a few samples of highly calcareous slates, but no continuous beds of limestone. 

 The same group, with the same mineral structure, and in the same position, but with 

 out organic remains, is greatly developed in Cumberland. The Lower Cambrian 

 group occupies the southwest coast of Caernarvonshire and a considerable portion of 

 Anglesea ; it consists chiefly of chlorite schist, passing here and there into mica schist 

 and slaty quartz rock and contains subordinate masses of serpentine and white gran- 

 ular limestone. It contains no organic remains. 



In this first description, Arcrean rocks are included in the lower 

 division ; the Middle Cambrian embraces the roofing slates, and the 

 Cambrian group as now generally accepted; while the Upper Cambrian 

 included much of the Lower Silurian of Sir 11. I. Murchison, or th< 

 Ordovician of Prof. Charles Lap worth. 



At a meeting of the Geological Society of London in the spring of 

 1838, Prof. Sedgwick read a paper entitled "A Synopsis of the English 

 series of stratified rocks inferior to the Old Bed sandstone, with an 

 attempt to determine the successive natural groups and formations." 1 

 AYhen describing the sections of North Wales, etc., the chlorite slates, 

 quartz rock, and mica slates of Anglesea and Caernarvonshire are not 

 included in the Cambrian as was done in 1835. 2 The Lower Cambrian 

 system includes the old slate series of Caernarvonshire and Merioneth- 

 shire, and ends with the calcareous beds, which range from Bala to the 

 neighborhood of DinasMowddy. 



The next group, the Upper Cambrian series, commences with the fossiliferous heels 

 of Bala. It includes all the higher portion of the Bcrwyns, and all the slate rocks 

 of South Wales, which are below the Silurian system. * * ' * Many of the fossils 

 are identical in species with those of the lower division of tho Silurian system, noi 

 have the true distinctive zoological characters of the group been well ascertained. 



In this arrangement, the Lower Cambrian of 1835 is referred to the 

 primary stratified group, to which he gave the provisional name of Pro- 

 tozoic. 3 



It is further stated that at the north end of the Berwyn chain th< 

 Upper Cambrian appears to pass by insensible gradations into tin 

 lower division of the upper system, the Caradoc sandstone. Prof. 

 Sedgwick does not appear to have been aware at this time that his Bala 

 series was a portion of Murchison's Lower Silurian, as described by the 

 latter in 1835. 



Many geologists have insisted upon restricting the Cambrian to the 

 Lower Cambrian of the scheme of 1835, which Prof. Sedgwick cut oi 



1 Proc. Geol. Soc, London, vol. 2, 1838, pp. 075-685. 



*Op. cit., p. 685. 



* Op. cit , p. 681. 



