M8 LOWER OOLITIC ROCKS OF ENGLAND : 
In most localities there is evidence of erosion on top of the 
Upper Lias clay, and the base of the Northampton Sand contains 
pebbles of argillaceous limestone and ironstone, sometimes covered 
with Serpulae and Polyzoa or bored by Lithodomi. With these 
pebbles, as pointed out by Prof. Judd and Mr. Beeby Thompson, 
we occasionally find examples of Ammonites bifrons, evidently due 
to the destruction of Upper Lias beds with septaria and cement- 
stones. 
The Lower Estuarine Beds, as Prof. Judd has suggested, may 
zepresent in part the Lower Freestones of the Cotteswold Hills ; 
but owing to the absence of distinctive fossils, we are not in a 
position to slate the zones that may in places be represented in 
the sub-division. Over considerable tracts in North Oxfordshire 
and South Northamptonshire, these strata are directly overlaid by 
the Upper Estuarine Beds.* 
Prof. Judd has pointed out many instances of unconformity 
where the junction of the Upper and Lower Estuarine Series 
can be examined. " The bottom-bed of -the Upper Estuarine 
Series, whenever this formation is distinctly developed, is found 
to be a band of ironstone-nodules, and these always rest on an 
eroded surface " of the underlying strata. This band however 
is not always present, and it is difficult sometimes to separate 
the two Estuarine Series. Moreover some of the irregular 
junctions may be attributed, partially at any rate, to subsequent 
erosion or dissolution of the underlying strata. As an example 
of the appearance presented by the junction of these two series 
of beds, the illustration of a pit near the Race-course at 
Northampton was given by Prof. Judd. " Here we have, 
in the lower part of the pit, beds of well stratified white 
sand with vertical plant-markings, and sandrock (the latter 
quarried as a building-stone), passing downwards into a dark 
brown sandstone with a very thin representative of the North- 
amptonshire ironstone at its base. On the eroded surface of 
these beds lie the light-blue, and often highly carbonaceous, clays 
of the Upper Estuarine Series, with the very constant layer of 
aodules (' ironstone junction-band') at its base." (Fig. 48. See 
also Figs. 51, p. 188, and 53, p. 191.) 
" Bearing in mind the existence of an unconformity between 
these two series of estuarine beds, we are not surprised to find 
that, in the country to the north, a thick series of beds (the Lincoln- 
shire Oolite) comes in like a great wedge between them. Thus 
in the northern port of the county of Northampton, along the 
ralley of the Nene ; the succession of beds is the same as that 
which we have already pointed out as presented in the neigh- 
bourhood of Northampton, while along the valley of the Welland, 
and in the country to the westward and northward, we have 
the same series of beds, with the addition of a new forma- 
tion, to which the Geological Survey has given the name of the 
* Judd, Geol. Rutland, pp. 39, 90, 92 ; Sharp, (juart. Journ. Geol. Soc., vol. xxvi. 
f. 375. 
