38 THE LEWIS BROOKS MUSEUM. 



others, so that the character of the vegetation changed 

 many times. These changes were always in the direction 

 of higher organization, greater diversity of species, and a 

 more strict limitation in habitat. The recotd of such 

 changes we find in the fossil plants contained in the sedi- 

 mentary rocks. We do not know that any portion of the 

 crust of the earth, first formed after cooling, is now visible. 

 The oldest known rocks, the Laurentian, are certainly sedi- 

 ments deposited in water. 



We may divide the column of sedimentary rocks into 

 five grand divisions. Thev are as follows, beginning with 

 the oldest : I. The Protozoic, containing the Laurentian, 

 Cambrian and Silurian ages. IT. The Paleozoic, contain- 

 ing the Devonian, Carboniferous and Permian. III. The 

 Mesozoic, containing the Triassio, Jurassic and Cretaceous. 



IV. The Tertiary, containing Eocene, Miocene and Pliocene. 



V. The Modern, containing the Quarternary • and Recent. 

 Each of these is marked by a vegetation peculiar to it. The 

 oldest sedimentary rocks of the globe, the Laurentian, in- 

 cluding the Huronian, are not known to have contained 

 plants. These rocks are too much altered to exhibit them 

 in recognizable form. They contain, however, large amounts 

 of carbon, in the form of graphite or plumbago. A por- 

 tion of this, no doubt, originated in vegetation. In this 

 State we have examples of such graphitic rocks in a belt of 

 country running parallel, to and east of the Blue Ridge. 

 The vegetation, if it existed, must have been sea weeds. 

 The same may be said of the Cambrian vegetation. , This 

 latter formation exhibits certain fossils which are commonly 

 classed with the sea weeds, although this classification does 

 not rest on very certain data. The oldest certainly known 

 plants are certainly sea weeds from the lower Silurian. 

 These are so varied and general in type that their affinities 

 cannot be clearly made out. There is hardly any differen- 

 tiation of parts in them, and nothing like organs is shown. 

 So far as we knew, the land in the lower Silurian had not 



