10 LIAS OF ENGLAND AND WALES : 
portions of which have been subjected to local erosion. Occa- 
sionally there may be found small and more or less rolled nodules 
of impure and sometimes slightly phosphatic limestone, the for- 
mation of'which may have been contemporaneous. (See p. 72.) 
Pauses in deposition may be indicated by accumulations of 
different genera and species of organic remains, by phosphatic 
nodules and phosphatized fossils, and by burrows of Annelides and 
Lithodomi. The evidence of marine borers, however, cannot be 
taken to prove any great lapse of time. In some cases calcareous 
strata must have been consolidated rapidly, for we find Annelide- 
borings and Lithodomi in successive layers of oolite that belong 
to the same subdivision. Abrupt changes in the character of 
sedimentary accumulations may occasionally indicate a break, but 
such evidence cannot be relied upon. We find throughout the 
Jurassic series frequent alternations of clays with sandstones or 
limestones ; changes that afford no evidence of any break in the 
sequence of events, but which may be attributed to variations in 
the currents bringing sediment. Even when such sedimentary 
changes are accompanied by changes in the character of the fossils, 
we have to consider to what extent the nature of the sea-bottom 
has influenced the forms of life. Marked differences in the suc- 
cessive assemblages of fossils, and a knowledge that elsewhere 
other fossil if erous strata are developed, afford (in the absence of 
distinct unconformity) the means by which local breaks of 
importance may be inferred. 
In a great series of strata that exhibits considerable sedimentary 
changes, it is natural to rind many local gaps due to paucity of 
sediment or to contemporaneous erosion. On account of these 
local unconformities and overlaps, different classifications of 
the strata may bo made in different localities. There is evidence 
in some places of unconformity between the minor subdivisions 
of one formation such as the Lo\ver Lias, the Inferior Oolite, or 
the Portland Beds. Thus there may be a break in the midst of a 
formation, while locally it may shade upwards and downwards 
into the overlying and underlying formations. 
Lateral Changes in Formations Stratiaraphical and 
Palceontoloaical Subdivisions. 
Having considered the vertical changes in the strata we may 
briefly allude to the lateral changes. In some cases, formations 
like the Inferior Oolite, the Great Oolite, and Forest Marble 
undergo considerable modifications as the beds are traced from 
place to place ; so that ultimately one subdivision, like the Fullers 
Earth or Forest Marble, may become replaced or absorbed by 
another stratigraphical division. In other cases the division may 
taper away, not having been deposited further, or it may have 
been denuded. 
These changes in the thickness, lithological character, and 
extent of the larger Jurassic division?, are but exaggerations of 
what takes places in the minor divisions. A formation may be 
