SUBDIVISIONS OF JURASSIC ROCKS 7 
other times of beds varying greatly in their component materials, 
: but yet united by some common character. Their order of super- 
position was clearly proved, and their organic contents were shown 
to be more or less distinctive. It is important to bear these facts 
in mind, for ail " formations " or stratigraphical divisions must, 
as far as possible, be regulated on these principles. 
The progress of knowledge has, however, shown that, when 
traced across the country from the south-west to the north-east, 
many of Smith's divisions, and especially the sandy and calcareous 
strata of the Oolites, exhibit marked changes in character. Hence 
it has been necessary to adopt distinct stratigraphical subdivisions 
for some of the formations in the midland and north-eastern 
counties. In no case can these divisions be taken as of equal or 
approximately equal value in point of duration or physical history : 
they are made essentially for convenience, and it must be remem- 
bered that all are subject, often to a considerable extent, to variation 
in thickness. 
The Jurassic system (Juraformation) of Germany was separated 
into three divisions in 1837 by Leopold Von Buch ; but the several 
stages have been somewhat differently grouped by geologists, 
more especially with regard to the Middle and Upper Jurassic 
divisions. The classification given in the Table on page 8, is that 
most commonly adopted. 
Sequence of rocks and Passage-beds Planes of demarcation. 
The Jurassic rocks represent for the most part marine accumu- 
lations that were nowhere deposited in very deep water, nor at a 
very great distance from the old lands to whose waste the detrital 
materials are due. The finer and more tranquilly deposited 
materials, like the clays, are more persistent than the current- 
bedded oolitic and sanely accumlations that were deposited in 
shallower water.* 
In certain formations, or subdivisions, only particular conditions 
of the sea-bed are presented to our view ; witli others we find 
evidences of the deposits of deeper and shallower water. In few 
cases are there preserved among Jurassic rocks the marginal 
accumulations that fringed the Palaeozoic lands, and in no instances 
do we find any abysmal deposit?. Estuarine characters appear in 
some of the beds, and these as a rule are shown in the character 
of the organic remains and in the nore abrupt changes in the 
sediments : impure coal-seams are found, carbonaceous shales and 
sandstones become more prominently developed, variegated clays 
occur, and the limestones are less conspicuous. More especially 
do these characters prevail, at certain horizons, as we trace the 
beds northwards, but, at the top of the series, the Purbeck beds 
of the south of England exhibit some entirely freshwater accumu- 
lations, as well as actual evidences of land-surfaces. 
That there was a continuous sequence of deposits in the British 
area throughout the Jurassic period, is shown by evidence obt lined 
* See Hull, Quart. Journ. Geol. Soc. vol. xvi. p. 72. 
