1274 ANNUAL OF SCIENTIFIC DISCOVERY. 



nature are dependent upon organization ; plants by solar force con- 

 vert water and carbonic acid into hydrocarbonaceous substances, from 

 whence bitumens, coal, anthracite and plumbago ; and it is the action 

 of organic matter which reduces sulphates, giving rise to metallic sul- 

 phurets and sulphur. In like manner it is by the action of dissolved 

 organic matters that oxide of iron is partially reduced and dissolved 

 from great masses of sediments, to be subsequently accumulated in 

 beds of iron ore. We see in the Laurentian series beds and veins 

 of metallic sulphurets, precisely as in more recent formations, and the 

 extensive beds of iron ore, hundreds of feet thick, which abound in 

 that ancient system, correspond not only to great volumes of strata 

 deprived of that metal, but, as we may suppose, to organic matters, 

 which but for the then great diffusion of iron oxide in conditions fa- 

 vorable for their oxidation, might have formed deposits of mineral 

 carbon far more extensive than those beds of plumbago which we ac- 

 tually meet in the Laurentian strata. 



All these conditions lead us then to conclude the existence of an 

 abundant vegetation during the Laurentian period ; nor are there 

 wanting evidences of animal life in these oldest strata. Sir William 

 Logan has described forms occurring in the Laurentian limestone 

 which cannot be distinguished from the silicified specimens of Stro- 

 matopora rugosa found in the Lower Silurian rocks. They consist 

 of concentric layers made up of crystalline grains of white pyroxene 

 in one case and of serpentine in another, the first imbedded in lime- 

 stone and the second in dolomite ; we may well suppose that the re- 

 sult of metamorphism would be to convert silicified fossils into sili- 

 cates of lime and magnesia. The nodules of phosphate of lime in 

 some beds of the Laurentian. limestones also recall the phosphatic 

 coprolites which are frequently met with in Lower Silurian strata, 

 and are in the latter case the exuviae of animals which have been fed 

 upon Lingula, Orbicula, Conularia, and Serpulites, the shells and 

 tubes of which we have long since shown to be similar in composition 

 to the bones of vertebrates. So far therefore from looking upon the 

 base of the Silurian as marking the dawn of life upon our planet, we 

 sec abundant reasons for supposing that organisms, probably as 

 varied and abundant as those of the palaeozoic age, may have existed 

 during the long Laurentian period. 



The Potsdam sandstone of the New York geologists is unquestion- 

 ably the lowest rock from below Quebec to the island of Montreal, 

 and thence passing up the valley of Lake Champlain, and sweeping 

 around the Adirondack Mountains, it re-enters Canada and disappears 

 to the north of Lake Ontario. In the valley of the Mississippi a 

 sandstone exists which is believed to be the equivalent of the Pots- 

 dam. 



The Potsdam sandstone of the St. Lawrence valley has for the 

 most part the character of a littoral formation, being made up in 

 great part of pure quartzose sand, and offering upon successive beds 

 ripple and wind-marks, and the tracks of animals. Occasionally it 

 includes beds of conglomerate, or encloses large rounded fragments 

 of green and black shale ; it also exliibits calcareous beds apparently 

 marking the passage to the succeeding formation. 



We may suppose, says Mr. Hunt, that while the Potsdam sandstone 



