EAHLY PALAEOZOIC STRATA. 233 



vonian formations. In some places these formations are of 

 great thickness; and they consist chiefly of clay-slate, of breccia 

 (Grauwacke), and limestone. The Cambrian and Silnrian for- 

 mations contain only marine organisms. About twenty species 

 of plants are known, all of which seem to belong to the Algae. 

 The precise nature of Oldhamia and Murchisonites cannot yet 

 be regarded as settled. Of animals, small Brachiopods and 

 Trilobites appear in the upper strata of the Cambrian series. In 

 the Silurian they are developed in a multiplicity of forms ; but 

 the same species in part occur in high northern and more 

 southern latitudes in North America and Europe, and even in 

 China. The Trilobites, which are the earliest known Articulate 

 animals, and belong to the Crustacea, disappear entirely in the 

 mountain-limestone of the Carboniferous formation, whilst the 

 Brachiopods have continued in existence to the present day; 

 indeed one genus (Lingula), a species of which now burrows 

 into the sand of tropical coasts, was represented even in the 

 seas of the Cambrian epoch, showing us that certain types of 

 animals are continued through all the ages of the world's his- 

 tory. Most of the Brachiopods of the early Palaeozoic rocks 

 belong to genera which do not now exist. Besides the Brachi- 

 opods, the Cephalopods are the Mollusca that occur most fre- 

 quently in these ancient seas. We find them with straight and 

 twisted shells, the septa of which are simple like those of the 

 Nautilus. In the Silurian strata they offer a wonderful multi- 

 plicity of species. In these deposits corals also make their 

 appearance, even at such an early period taking part in the 

 formation of the crust of the earth, and forming reefs in which 

 numerous crinoids, very simply constructed Sea-urchins, and 

 Sponges found a dwelling-place. Fishes first appear in the 

 uppermost Silurian stratum; in the upper Devonian deposits 

 their species are tolerably numerous. All the forms, however, 

 differ greatly from those now existing, and they in part form 



and as composed of felspathic rocks inferstratified with important masses of 

 limestone and quartzite." The area in Canada occupied by the Laurentian 

 eerie.? is supposed to be about 200,000 square miles. EDITOR.] 



