20 LIAS OF .ENGLAND AND WALES: 
though a single characteristic species may furnish a reliable clue 
to the general horizon. 
Where precise divisional planes are taken between zones, it is 
where some more or less marked lithological change takes place ; 
but such divisions have at most but a l.ical value. Where 
different zones occur in a series of clays that present no 
prominent lithological distinction, as is the case with the zones 
belonging to the Lower and Middle Lias Clays, the Upper Lias, 
the Oxford and Kimeridge Clays, only approximate boundary-lines 
can be affixed. Nevertheless the sequence of different assemblages 
can be followed better in the great deposits of clay and in the 
limestones of more or less detrital and sedimentary origin, than in 
the false-bedded oolites and sandy strata where the beds are im- 
persistent and the fossils less frequently and less perfectly pre- 
served. 
It will be generally admitted that zones, occurring as they do 
irrespective of sedimentary conditions, and comprising a number ot 
species with varying geographical and geological ranges, have no 
exact limitations ; and thus the requirements of the geologist can- 
not always be reconciled with the demands of those who would 
subdivide our strata purely on palseonlological evidence. 
The great value of zones is in marking the sequence of organic 
remains, and in Furnishing material for correlating, in a broad way, 
strata that are far apart. Moreover these palseontological divisions 
furnish convenient horizons for those engaged in working out the 
biological history of species. 
Comparisons between the British Jurassic strata and those met 
with on the Continent and elsewhere will be left for the final 
volume.* It will be found that distinct stratigraphical divisions 
must be made in different areas, while the main palseontological 
horizons will be found to correspond in a remarkable degree over 
wide areas. Among the numerous zones it has been sought to 
establish, many have but a local importance, losing their individu- 
ality among the strata over more extended tracts. Such minor 
zones or "sub-zones" are sometimes useful, but there is practically 
no end to the number that might be made. 
* See Table by C. Fox-Strangways, Jurassic Rocks of Yorkshire, vol. i. p. 21. 
