235 



black carbonaceous or bituminous mass. lu passing biglier up, in beds 

 of the same period, the forms become more distinct and less disfigured 

 by compression, but mostly remain simple, however, like narrow, cylin- 

 drical, rigid, or flexuous stems, without branches and with smooth sur- 

 faces. These characters seem to indicate a simple structure of the Algse 

 by juxtaposition of elongated cells joined by their ends, as are now the 

 thread-like filaments of the thermal springs. In ascending still higher 

 — to the Upper Silurian — these fucoids appear more diversified : they bear 

 branches ; their surface is wrinkled or striated in many ways ; and their 

 characters being thus more distinct and multiplied, they are open to 

 analysis and to classification. Already a number of them have been 

 described from these strata. At the same time, the fossil remains in- 

 crease in number to such a degree that strata of shale or of limestone 

 seem, locally, to be a compound of fragments of sea-weeds. As these 

 petrified plants apparently represent only the easily-preserved species, 

 those of a coriaceous hard tissue, the profusion of their remains may give 

 an idea of the exuberance of the marine flora during the more ancient 

 periods of our earth. It was evidently still greater than at the present 

 time, though the activity of this vegetation is now manifested to a high 

 degree by the heaps of the bladder- weed and other species along our 

 shores, as also in mid-ocean by banks of the floating Sargassum, covering 

 thousands of square miles, and thick enough to impede or almost stop 

 the progress of ships. The fucoids of old, some of which were ot great 

 size, though composed only of vascular tissue and without woody fibers, 

 foreshadowed the appearance of the coal-plants, to which they already did 

 play in the economy of nature a somewhat analogous part. From their 

 decomposition have resulted the deposits of bitumen, or mineral oil, 

 which man's ingenuity uses now to an advantage not equal indeed, but 

 comparable, to that which he derives from the coal. 



The remains of sea-weeds do not give any indication of the tempera- 

 ture or the atmospheric circumstances governing our globe during 

 that long Silurian period when water covered most of the surface of 

 the earth, eitlier in a condensed form as fluid, or as vapor. The tem- 

 perature of the sea was of a higher degree, evidently, than it is at the 

 present time even under the influence of tropical heat. The simple 

 structure of the sea-weeds and their cylindrical form seem to indicate 

 this physical fact by the coincidence, remarked above, of coufervoid 

 plants of simple and analogous conformation found now in the hottest 

 thermal water. In the whole thickness of the rocks formed during this 

 long period, which in some countries, as in England, attain a thickness 

 of 60,000 feet, no traces of land-plants had been previously observed. 

 A few cylindrical branches, or impressions of branches upon clay, bear- 

 ing upon their surface rhomboidal scars disposed in spirals around the 

 stem like those of Lepidodendron, were lately discovered in the Cincin- 

 nati group of the Middle Silurian. As they were found associated in 

 the same strata with fragments of fucoids and deep marine mollusks, their 

 relation was considered as being rather with peculiar forms of Algi3e. 

 More recently, however, fossil remains of two species of vegetables, 

 positively recognized as land-plants, have been found in the Silurian 

 formation of the Lower Helderberg of Michigan, and attest the 

 existence of land plants, and, consequently, animal life also, in the 

 Silurian period, a fact which till now had remained uncertain. The 

 presence of land-plants in strata of a lower formation — that of the Cin- 

 cinnati group — becomes less improbable by this discovery. 



In the whole thickness of the following period, the Devonian, the 

 marine vegetation is still predominant, as testified by fucoidal remains 



