Ch. XXVII.] CAMBRIAN" GROUP. 573 



down to the Calciferous Sandstone inclusively with the Upper and 

 Lower Llandeilo, we shall be in harmony with the latest opinions of 

 American and British geologists. 



In Canada, as in the State of New York, the Potsdam Sandstone 

 underlies the above-mentioned calcareous rocks, but contains a dif- 

 ferent suite of fossils, as will be hereafter explained. In parts of the 

 globe still more remote from Europe the Silurian strata have also been 

 recognized, as in South America, Australia, and recently by Captain 

 Strachey in India. In all these regions the facies of the fauna, or 

 the types of organic life, enable us to recognize the contemporane- 

 ous origin of the rocks ; but the fossil species are distinct, showing 

 that the old notion of a universal diffusion throughout the " primeval 

 seas " of one uniform specific fauna was quite unfounded, geographical 

 provinces having evidently existed in the oldest as in the most modern 

 times. 



Whether the Silurian roclcs are of deep-water origin. — The grounds 

 relied upon by Professor E. Forbes for inferring that the larger part 

 of the Silurian Fauna is indicative of a sea more than 70 fathoms 

 deep, are the following : first, the small size of the greater number of 

 conchifera ; secondly, the paucity of pectinibranchiata (or spiral uni- 

 valves) ; thirdly, the great number of floating shells, such as Bellero- 

 jrfion, Orthoceras, &c. ; fourthly, the abundance of orthidiform brachi- 

 opoda ; fifthly, the absence or great rarity of fossil fish. 



It is doubtless true that some living Terehratulce, on the coast of Aus- 

 tralia, inhabit shallow water ; but all the known species, allied in form 

 to the extinct Orthis, inhabit the depths of the sea. It should als,o be 

 remarked that Mr. Forbes, in advocating these views, was well aware 

 of the existence of shores, bounding the Silurian sea in Shropshire, and 

 of the occurrence of littoral species of this early date in the northern 

 hemisphere. Such facts are not inconsistent with his theory ; for he 

 has shown, in another work, how, on the coast of Lycia, deep-sea 

 strata are at present forming in the Mediterranean, in the vicinity of 

 high and steep land. 



Had we discovered the ancient delta of some large Silurian river, 

 we should doubtless have known more of the shallow-water, brackish- 

 water, and fluviatile animals, and of the terrestrial flora of the period 

 under consideration. To assume that there were no such deltas in 

 the Silurian world, would be almost as gratuitous an hypothesis, as for 

 the inhabitants of the coral islands of the Pacific to indulge in a similar 

 generalization respecting the actual condition of the globe. 



" CAMBRIAN GROUP." 



[Primordial Zone of Barrande.) 



The characters of the Upper and Lower Silurian rocks were estab- 

 lished so fully, both on stratigraphical and palaeontological data, by 



