THE AVONIAN OP THE AVON GORGE 
81 
by reducing the thicknesses of various deposits, by estimation, 
to one uniform rock-type. 
A Range-diagram is now constructed by plotting all the 
gens-lines parallel to one another, and at arbitrary distance 
apart, in such a manner that any transverse line which cuts 
all the parallels at right angles is an isochrone, and the 
aggregate of the mutations which lie upon it represents the 
faunal assemblage of a particular instant of zonal- time. 
The Range-diagram is the complete expression of the faunal 
succession and, upon it, every system of zoning must be 
founded. 
It is not, however, an entirely simple matter to draw up a 
series of zones from the mere inspection of a Range-diagram. 
Theoretically, any one of the long gens lines upon which 
several stations are marked would serve as a time-scale, but 
the longer the life of a gens, the greater is its stability and, 
consequently, the greater are the distances between successive 
mutations and the smaller the actual difference between them 
(in other words, a long-lived gens has a low variation-gradient). 
Orthothetes cf. crenistria ahords an excellent example of the 
practical difficulty in employing this method. 
Again, a long-lived gens is usually dominant only during a 
limited part of its existence and is relativel}^ unimportant 
during the remainder of its life. 
The practical stratigrapher naturally desires to fix his zonal 
position by the aid of fossils which are at once easily found and 
easily recognized (in fact by a system of ‘ spot- zoning ’) and 
this method leads to absolutely accurate results, so long as it 
is only relied upon within a small area. There are, however, 
several weighty objections to this method, if its intrinsic limita- 
tions are not fully appreciated. 
(1) It usually happens that the various species of a gens, or 
of closely allied gentes, are not all dominant throughout a large 
area, but that an earlier species is dominant in one part and 
rare in another, whereas a later species may be rare in the first 
locality and dominant in the second. Hence, the field worker, 
