THE CAMBRIAN PERIOD. 



79 



formation. In Bohemia, the Upper Cambrian, in particular, 

 is largely developed, and constitutes the so-called " Primordial 

 zone" of Barrande. Lastly, in North America, whilst the 

 Lower Cambrian is only imperfectly developed, or is repre- 

 sented by the Huronian, the Upper Cambrian formation has 

 a wide extension, containing fossils similar in character to the 

 analogous strata in Europe, and known as the " Potsdam Sand- 

 stone." The subjoined table shows the chief areas where Cam- 

 brian Rocks are developed, and their general equivalency : 



TABULAR VIEW OF THE CAMBRIAN FORMATION. 



Like all the older Palaeozoic deposits, the Cambrian Rocks, 

 though by no means necessarily what would be called actually 

 " metamorphic," have been highly cleaved, and otherwise 

 altered from their original condition. Owing partly to their 

 indurated state, and partly to their great antiquity, they are 

 usually found in the heart of mountainous districts, which have 

 undergone great disturbance, and have been subjected to an 

 enormous amount of denudation. In some cases, as in the 

 Longmynd Hills in Shropshire, they form low rounded eleva- 

 tions, largely covered by pasture, and with few or no elements 

 of sublimity. In other cases, however, they rise into bold and 

 rugged mountains, girded by precipitous cliffs. Industrially, 

 the Cambrian Rocks are of interest, if only for the reason that 

 the celebrated Welsh slates of Llanberis are derived from 

 highly-cleaved beds of this age Taken as a whole, the Cam- 

 brian formation is essentially Composed of arenaceous and 



