LIASSIC ZONES. 27 
particular species within certain ascertained limits. So far as 
possible these zones will be indicated in the conrse of this work, 
for they may be useful in instituting comparisons with strata else- 
where ; but it must be clearly borne in mind that the non- 
recognition of any zone, owing to the absence of the index-species, 
is no proof of any break in the succession. 
Zones marked by one species may be represented locally by 
a different, and perhaps closely allied form, as in the case of 
Ammonites capricomus an'l A. Henleyi, of A. Turneri and A. 
semicostatus, or of A. planorMs and A. Johnstoni, &c. 
The inability to define the stratigraphical limits of zones in a 
series of comparatively uniform clays, renders it difficult in many 
cases to prove their continuity, and it may be admitted that the 
strata assigned to particular zones in different parts of the country 
cannot be regarded as precisely or entirely synchronous. It is 
however enough to admit the general contemporaneity of beds, 
without attempting to fix particular limits for sediments that were 
continuously deposited. 
It is indeed remarkable that throughout the Lias, which is 
essentially an argillaceous formation, with here and there more or 
less marked bands of limestone and with local divisions of sandy 
strata, there is such a marked change in the forms of Ammonites, 
not to mention other species, that are met with at successive 
stages. In the higher divisions of the Lower Lias and the lower 
portion of the Middle Lias there is an unbroken series of clayey 
sediments, and we have no reason to conclude that their deposition 
was other than tranquil and uniform over considerable areas. 
Yet we find a tolerably distinct series of Ammonites coming on 
in succession, and other changes in the fauna, independent of the 
conditions of sedimentation.* Such a repetition of similar con- 
ditions attended by gradual changes in the character of the fauna 
is met with in the Chalk. It seems probable that the changes 
were brought about by slow, and perhaps occasionally by rapid, 
subsidence, connecting areas previously separated to a greater or 
less extent ; whereby different species became introduced, and in 
some cases those already existing in the area may have become 
modified. Physical changes that affected the food-supply of 
Mollusca no doubt influenced the forms of life. Moreover species 
that had a prolonged individual existence became modified in 
their old age, so that certain so-called species are admitted by 
some authorities to represent stages of growth as in the case of 
Ammonites capricomus and A. Henleyi ; while many other species 
are linked together by intermediate or l< exallagous " forms to 
which the name of " mutations " is given. f 
Formation of the Rocks. 
There are many stratigraphical facts that lead to the conclusion 
that the Lias limestones were to a considerable extent sedimentary 
* See also R. Tate, Quart. Journ. Geol. Soc., vol. xxvi. p. 401 ; Tate and Blake 
Yorkshire Lias, p. 217 ; and H. B. W., Proc, Geol. Assoc , vol. xii. p. 306. 
f See J. F. Blake, Proc. Geol. Assoc., vol. xii. p. 277. The word " exallagous " 
was introduced by J. E. Marr, Natural Science, April 1892, p. 125. 
