I'ASSAGE-BEUS. 9 
in different localilies ; so that notwithstanding the physical changes 
indicated by the successive deposits of deeper and shallower water, 
yet some portions o the area \\ere receiving sedimentary deposits 
during the period. 
The evidences of transition will be pointed out in the chapters 
dealing with the Lias and Oolites; they are seen in Passage-beds 
that show the gradual passage of one formation of particular litho- 
logical character into another of dissimilar nature, or the alternation 
of beds at the summit of one formation with those of a type 
characterizing the succeeding deposit. The classification of these 
Passage-beds, which serve to unite formations accumulated under 
varying sedimentary conditions, has led to much debate of an 
unprofitable character. While, as Sedgwick remarked,* these 
evidences of conformity indicate "the perfect development of the 
series," yet we find abundant evidence of the more ample, or it 
may be said the more eventful, development of a formation at one 
locality than another. This may be marked by rich fossil-beds, 
as well as by a greater thickness of deposits, due to more rapid 
deposition of sediment. Hence it happens that some localities, 
originally chosen to designate formations, are not always the best 
that could have been selected ; but it is obvious, that when the 
meaning of a term is well understood, no advantage can result from 
a change. 
In certain localities, therefore, it is not possible to fix precise 
lines of demarcation between subdivisions ; this is the case with 
the Upper Lias Clay and Midford Sands, the Great Oolite and 
Forest Marble, as well as other formations. Again in many 
localities the junctions of formations may be well-marked ; as is 
often the case where the Lower Lias rests on the White Lias 
(Kha3tic), the Upper Lias clay on the Marls tone, or the Fuller's 
Earth clay on the Inferior Oolite. 
The completeness of the Jurassic record is interrupted in many 
instances by local erosion and by un conformable overlap ; but 
there is no evidence of great discordance between the stratifica- 
tion of the unconforrrmble strata, such as would indicate very 
considerable local disturbances. Nowhere in Britain do we find 
any evidence of contemporaneous volcanic activity ; and only in 
Yorkshire, the north-east of Ireland, and along the western side 
of Scotland, are the beds affected by the subsequent intrusion of 
eruptive rocks. 
Evidences of local erosion are shown by the presence of pebbles 
of previously formed Jurassic rock?. Among the current-bedded 
oolites the occurrence may be noted of fragments of strata formed 
not long previously, or of portions even of the same subdivision. 
Thus we find rolled pieces of oolite in some of the beds of the 
Inferior Oolite. Again we find rolled pieces of cement-stone or 
ironstone at the base of the Inferior Oolite ; and in this case the 
pebbles have been formed from a distinct formation, the higher 
* Ann. Phil 1820, vol. xxvii. p. 350. 
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