20 
SILTJKIA. 
[Chap. I. 
endeavour will be made, in the Nineteenth Chapter, to show that gold, 
however it may now be spread over the surface, has been introduced most 
abundantly into the older deposits, and chiefly into those of Lower Silurian 
or older date, and has also accompanied certain eruptive rocks of later 
date. 
Lastly, it is to be observed that, as the true succession of the chief 
masses of the earlier fossiliferous strata was first detected in England and 
Wales, so the geological descriptions in this volume will in the first instance 
be derived from our typical insular examples. A general comparison will 
afterwards be instituted with the contemporaneous rocks in different quar- 
ters of the globe. 
The importance of having, through patient surveys, mastered the obscu- 
rities which clouded the history of the earlier periods of organic life will 
thus, it is hoped, be rendered obvious, by showing that we have now ob- 
tained as correct an insight into the primeval fossil-bearing formations as 
we had previously acquired of the nature and history of the younger de- 
posits. 
