- 
262 Seeley— Sequence of Rocks and Fossils. 
VI. On THE SIGNIFICANCE OF THE SEQUENCE OF Rocks AND Fos- 
SILS : THEORETICAL CONSIDERATIONS ON THE UPPER SECONDARY 
Rocks, AS SEEN IN THE SECTION AT ELy.* 
By Harry SEExey, Esq., F.G.S., Woodwardian Museum, Cambridge. 
(ees are generally the mud of rivers ; sandstones, the detritus of 
old crystalline rocks; while limestones are organically or chemi- 
cally formed. If, therefore, there is a great clay, it bespeaks a great 
river draining lands that were; though small and local clay-bands 
may be but the ruins of denuded sea-cliffs. The sandstone is the 
evidence of coast and reef, where weather and wave disintegrated 
plutonic rocks, or constructed over again the paleozoic or older sand- 
rocks. And limestones have been formed either where the land 
had sunk, or where the material was derived from the denudation 
of calcareous cliffs in shallow seas. 
The physical significance of many beds may be gleaned from ap- 
plication of these principles ; and in a rude way an approximation 
may be made not only to the relations of land and water in former 
times, but to the order and places in which changes of level were 
made. 
It is worth notice, that while the Cretaceous clay alternates with 
sandstones, the Oolitic clays for the most part alternate with lime- 
stones. These Lower Secondary limestones appear to have been 
partly derived from pre-existing calcareous beds; partly to be owing 
to rivers flowing through limestones; and are partly organic. So, 
as will be seen, they indicate features of physical geography unlike 
what afterwards obtained in the Upper Secondary strata; and it 
seems that important changes of level, bringing a new series of rocks 
into wear, marked the incoming of the Cretaceous rocks and fossils. 
To understand the lithology and paleontology of any bed, it is 
necessary to remember why the deposit exists; since it may include 
several different rocks; or different deposits may be included under 
the same rock. This may be illustrated by some old physical changes 
in the Cambridgeshire district. 
Under that northern extension of the Upper Greensand, the 
Hunstanton Red Rock, there is a deposit, which at Speeton and 
Huns’ton rests on the Kimmeridge Clay. At Speeton this is the 
Speeton Clay, which, Professor Phillips assures me, represents con- 
tinuously in one stratum the Shanklin Sands and the Gault. And 
the Huns’ton Rock, which is there aluminous, being above, the whole 
succession is perfect to the Chalk. Now at Huns’ton, instead of a 
clay between the Red Rock and the Kimmeridge Clay, there are 
sands; and the Red Rock, instead of being aluminous, is sandy. The 
fossils of this Carstone, as it is called, are few. ‘Those from its base 
might well belong to the Shanklin Sands; but species from the 
upper part, like Ammonites splendens, are such as would rather be 
associated with the Gault. It is therefore far from unlikely that at 
* This paper was read March 7, 1864, before the Philosophical Society of Cam- 
bridge. é 
