458 



SPECIAL GEOLOGY. 



The accompanying illustration contains figures of a few of 

 the most prevalent Silurian shells. (Pig. 300.) 



FIG. 300. 1. Orthis orbicularia. 2. Terebratula navicnla. 3. Orthis navicularis 

 4. Pentamerus Knightii. 5. Atrypa affinis. 6. Phragmoceras ventricosum. 

 7. Lituites giganteus. 



CAMBEIAN AND CTJMBEIAN GEOTTPS. 



THE CAMBRIAN G-EOTJP consists of a vast series of rocks, 

 of a slaty character, in some of which fossils have been found. 

 The iipper beds of Snowdon contain two species of corals 

 (Cyathophyllum) and six of the older forms of Terebratula. 

 Professor Sedgwick divides this group into the 



Plynlymmon rocks. Argillaceous indurated rocks, sandy 

 or slaty. 



Bala limestone. Dark laminated limestone and slate. 

 Snowdon rocks. Pine and coarse-grained slaty rocks. 



THE CUMBRIAN GEOUP. The Possiliferous slates of the 



