THE SUCCESSION OF STRATI FIKH KOCK8 7 



In many respects those names which have a geographical or 

 historical origin are the most satisfactory names taken from those 

 of towns, rivers, mountain ranges, provinces, or the ancient races 

 which inhabited such provinces ; as Jurassic (from the Jura range), 

 Devonian (from the county of Devon), Cambrian (from the Roman 

 name of North Wales), Silurian (from the tribe of the Silures). 

 These will probably remain as permanent names for systems to 

 which they are applied. 



Definition and Delimitation of Systems. The number 

 of systems into which the geological series should be divided, and 

 the horizons at which the planes of separation are placed, are points 

 requiring some consideration, because they involve certain principles 

 of classification. The divisional lines are generally drawn where 

 natural breaks occur, either in the sequence of the strata themselves, 

 or in the succession of fossil species which they contain. Such 

 breaks are therefore of two kinds, (1) Stratigraphical, (2) Pal. 

 logical. 



1. Stratigraphical Breaks. The nature of Stratigraphical breaks 

 or unconformities is explained in all handbooks of physical 

 geology, and in the present connection we have to deal only 

 with their relative importance and value in classification. The 

 most complete and satisfactory measure of any unconformity is 

 the thickness of strata which were deposited in other areas during 

 the time represented by the gap between the two unconformable 

 formations. 



It is not every unconformity which is of sufficient importance 

 to form a plane of division between two systems. Thus in Wales 

 there is a great local unconformity within the Silurian System, bat 

 this does not exist in other areas. Again, in Northampton and 

 Lincolnshire there is unconformity between two portions of the 

 Jurassic System, while no such break exists either in the south of 

 England or in Yorkshire. 



A break which is represented elsewhere by several thousand 

 feet of rock implies the lapse of a considerable period, and being 

 always accompanied by a great change in the fossils, might be taken 

 as a line of division between two systems. But if systems were to 

 be rigidly limited by physical unconformities, and defined as a 

 series of beds formed during a complete oscillation of downward 

 and upward movement, the classification of the rocks in one area 

 will break down when applied to those of other districts, because 

 the gaps will occur at different horizons in different areas. Thus 

 in Britain alone three different classifications at least wouM be 

 required for the Primary rocks, as the following columns will show, 

 a being taken to mean a break or unconformity. 



