o 



08 THE CANADIAN NATURALIST. [Yol. VI. 



Upper Cambrinn of Sedgwick ; a concession whicli can hardly 

 be defended, but which apparently found its way into use at 

 a time when the yet unravelled perplexities of the Welsh rocks 

 led Sedgwick himself to propose, for a time, the name of Cambro- 

 Silurian for the Bala group. This want of agreement among 

 geologists as to the nomenclature of the lower paleozoic rocks, 

 causes no little confusion to the learner. AVe have seen that 

 Henry Darwin Roirers followed Seduwick in Giving; the name of 

 Cambrian to the whole paleozoic series up to the base of the May 

 Hill sandstone; and the same view is adopted by Woodward in 

 his M inu il of the Mollusca. The student of this excellent book 

 will find that in the tables eivinji' the j>eolo£iical ranoe of the 

 mollusca, on pages 124, 125 and 127, the name of Cambrian is 

 used in Seduwick's sense, as includina' all the fossiliferous strata 

 beneath the May Hill sandstone. On page 123 it is however 

 explained that Lower Silurian is a synonjm for Cambrian, and 

 it is so used in the body of the work. 



The distribution of the Lower and Middle Cambrian rocks in 

 Great Britain may now be noticed. The former, or Bangor group, 

 to which Murcliison and the Geological Survey restrict the name 

 cf Cambrian, and which they sometimes call the Longmynd, 

 bottom or basement rocks, occupy two adjacent areas in Caernar- 

 von and Merionethshire; the one near Baniior, includin2: Llan- 

 berris, to the uoith-east, and the other, including Harlech and 

 B irmouth, to the south-east of Snowdon ; this mountain lying in a 

 synclin;-l between them, and rising 3571 feet jibove the tea. The 

 great mass of grits or sandstones appears to be at the summit of 

 the group, but in the lower part the blue roofing-slates of Llan- 

 beiris are interstratified in a series of green and purple slates, 

 grits and conglomerates. (Some of the Welsh roofing-slates are 

 hovN'Gver supposed to belong to the Llandeilo). [Mem. Geol. 

 Survey III, part 2, pages 54, 258.] The Harlech rocks in this 

 north-western region are conformably overlaid by the Mcnevian, 

 followed by the true Lingula-flag?*, or Olenus beds, of the Middle 

 Ci.mbiian. I'^pon these repose the Trcmadoc slates, which are 

 not known in the other parts of Wales. Tlie third area of Lower 

 Cambrian rocks is that already described at St. David's in Pem- 

 brokeshire, about 100 miles to the south-west; and the fourth, 

 that of the Longmynd hills, about sixty miles to the south-east 

 of Snowdon. The rocks of the Longmynd, like those of the 

 other Lower Cambrian areas mentioned, consist principally of 



