328 



PALEOZOIC SYSTEM OF ROCKS. 



which Silurian was of course so much marginal sea-bottom exposed by 

 upheaval during and at the end of Silurian times. There was also a 

 large island in the Devonian seas in the region about Cincinnati, viz., 

 the Silurian area, situated there (see map, p. 291). In the Plateau 

 region there was a large extent of land in later Silurian and Devonian 

 times, as shown by the absence of strata of these times in the Grand 

 Canon section. At the end of Devonian times the Devonian area was 

 added to the existing land, and the continental mass was further in- 

 creased. 



Subdivision into Periods. — In the United States the following five 

 periods are usually recognized : 



5. Catskill period. 



4. Chemung period. 



3. Hamilton period. 



2. Corniferous period. 



1. Oriskany period. 







We shall, however, neglect these subdivisions in our general de- 

 scription of the life of the age. 



Life-System of Devonian Age — Plants. 



It will be remembered that during the Silurian age, except a few 

 small vascular c^ptogams^ the only plants found were Fucoids. These 

 continued, though under different species, in Devonian times. But, in 

 addition to these, were now introduced land-plants in considerable 

 numbers and variety, and decided complexity of organization. They 

 included all the orders of vascular cryptogams, viz., Ferns, Lycopods, 

 and Equisetce ; and also Conifers among gymnospermous Phaenogams ; 



Fig. 389.— Microscopic Section of the Silicified 

 Wood of a Conifer (Sequoia^, cut in the 

 long direction of the fibers. Post-tertiary ? 

 Colorado (after Nicholson). 



Fig. 390. — Microscopic Section of tho Wood of 

 the Common Larch (Abies larix), cut in the 

 long direction of the fibers. In both the 

 fresh and the fossil wood (Fig. 389) are seen 

 the disks characteristic of coniferous wood 

 (after Nicholson). 



and by their great size and numbers probably formed for the first time 

 in the history of the earth a true forest vegetation. 



