CAMBRIAN AND SILURIAN. 



575 



Granite was at one time considered to be the true primitive rock. 

 Gneiss is a word of Saxon origin, and consists of the same materials as 



granite in different proportions. Mica- 

 schist is made up of two of the same 

 constituents as the granite and gneiss. 

 They are without fossil traces. 



The CAMBRIAN system of aqueous 

 origin may be said s to contain evidence 

 of the dawn of organic life. It is part 

 of the clay-slate system, and the term 

 " Cambrian " is taken from the ancient 

 name of Wales, where slate is plentiful. 

 Mica-slate is also very important. These 

 Cambrian rocks are of the next oldest formation to the Laurentian, and all 

 the various deposits may be examined in Wales, where also traces of volcanic 

 and ice action may readily be perceived. In the pass of Llanberis one 

 immense ice-borne block is very prominent ; no agency but ice could 

 have put it there as it rests. It is estimated that the Cambrian and 



Fig. 650. Nereites Cambrensis. 



Fig. 651 . Section across Snowdou. 



A, Fossiliferous grits (Bala series) : B, Greenstone (intrusive) ; C, Porphyry ; 

 D, Volcanic ashes, sometimes calcareous and fossiliferous Bala limestone. 



"Lower" Silurian rocks are from 20,000 to 30,000 feet in thickness, 

 and must embrace a very lengthened period. The fossils of these forma- 

 tions show that zoophytes and certain primitive Crustacea lived in the 

 remote ages when these rocks were formed by sedimentary deposition. 

 We have scarce a trace of plant-life. The organic remains include annular 

 worms, the first arrangement of the articulated animals according to Cuvier. 

 The flora and fauna are, of course, very low in the scale of creation, when 

 land and sea were so differently arranged. 



The SILURIAN system was so named by 

 Sir. R. Murchison, after the territory formerly 

 occupied by the Silures, but the system is, of 

 course, universal. We have here sandstones, 

 limestones, and shales, deposits lying upon the 

 Welsh slate. The Upper and Lower Ludlow 

 beds, and the " May Hill" sandstone, then the 

 Lower Silurian, with Caradoc beds, and the Fig. 6 52 .-siiurian fossil 



Tremadoc slate, etc. In this system volcanic action is observable, and all 

 the organic remains are those of marine animals, such as corals, shell-fish, 



