45 



after their deposition and their remaining so until the Old Red period 

 had passed ; or to the Old Red having been deposited on the Silurian, 

 the whole elevated above the sea and subjected to denudation sufficient 

 to remove the Old Red ; in either case when the land was again sub- 

 merged the formation then in process of deposit would be laid down 

 directly on the Silurian. One thing should be carefully noted, viz.» 

 that the rocks never occur in an inverted order, i.e., no one of the 

 Cambrian beds is to be found resting on any of the Silurian, nor any of 

 either of these on a representative of the Old Red. 



As each superimposed stratum is older than that underlying it, so 

 the fossils contained in an overlying bed are less ancient than those in 

 the bed beneath, and thus the succession of organic remains are the 

 evidences of the growth and development of living forms. 



As aforesaid, each formation and each of its subdivisions is charac- 

 terized by a peculiar assemblage of fossils, and, owing to this, when we 

 find the peculiar assemblage repeated, we know that we have a repetition 

 of the formation in which it was originally discovered ; and thus a 

 knowledge of the fossils contained in any particular bed or group of 

 beds, enables us to determine the relative age and position of the beds 

 compared with other beds in the same region or other regions. How 

 far this test may be applied with certainty to minor rock groups or 

 sub-divisions, we have not time here to consider, but its value to deter- 

 mine formations has never been questioned. Further, although the 

 combination or assemblage of fossils is peculiar to that formation or 

 sub-division, each fossil of it is not, for it may go on upward through 

 several formations before it disappears : one thing, remember, that 

 when it once disappears it never reappears. 



Let us suppose that in any one country we discover the order and 

 relative succession of the rocks, and that we have examined and noted 

 the mineral character of the beds as well as the fossils they contain. 

 Now to identify any outlying beds throughout the neighbourhood, the 

 mineral character alone would be a sufficient means of identification for 

 most of the beds, while superposition would do for the remainder. But 

 to determine the stratigraphical position of rocks in a far off region from 

 mineral evidence obtained in this country would be vain — here a com- 

 parison of the fossils would be the only reliable test. For instance, the 



