166 LOWER OOLITIC ROCKS OF ENGLAND: 
The LOWER ESTUARINE SERIES consists of white, brown, and 
grey sands with plant -remains, and " vertical markings " of their 
rootlets, with inconstant beds of clay, ferruginous nodules, &c. 
The beds are 10 or 20 feet thick. Cyrena occurs rarely ; and 
other fossils are seldom found. Like the Upper Estuarine Series, 
there is evidence of the alternation of ftuvio-marine, and terrestrial 
conditions ; while the Northampton Sand below is essentially 
marine. In many localities however it is not possible to fix any 
plane of division between the strata ; and where a boundary can 
be taken, it is likely to vary in horizon at different places. 
The NORTHAMPTON SAND comprises beds of hard calcareous 
sandstone, ironstone, and occasional oolitic sandy limestone ; these 
pass, often within short distances, into beds of white sand or iron- 
stone. As Prof. Judd remarks, " Sometimes the whole thickness of 
the Northampton Sand is made up of white sands with occasional 
beds of clay; . . . but in the majority of instances a greater 
or less portion of its mass, usually towards its lower part, is con- 
verted into a solid bluish or greenish ironstone rock of oolitic struc- 
ture, exactly resembling many parts of the Dogger and Middle 
Lias ironstones of Yorkshire ; this rock, by weathering action set 
up from its joint planes, assumes a bnrvvn colour and a banded or 
cellular structure of a very peculiar and striking character. 
. . This structure is due to the chemical action set up in 
the mass by the atmospheric waters, which, penetrating from the 
joint and bedding planes, have caused the concentration of 
hydrated peroxide of iron along surfaces having a general 
parallelism with those planes. The hard bands are often concen- 
trically arranged. Frequently the change by weathering from 
blue and green carbonate and silicate to brown haematite has only 
partially taken place, and the centres of the blocks consist of the 
former while their outer portions are constituted by the latter, 
displaying the usual hard bands. The brown ore, when examined 
microscopically, is often seen to retain the same oolitic structure 
which is found in the unweathered rock/'* (See Fig. 137, p t 494). 
The thickness of the Northampton Sand is variable ; as Pro- 
fessor Judd remarks, it probably never exceeds 40 feet, while it is 
frequently reduced to very insignificant proportions, and some- 
times, as about Luffenham, almost entirely disappears, being 
there represented by thin beds of white sand, clay 1 ! and ironstone. 
These beds apparently die out for a space east of Barrowden and 
Wakerley, where they are from 2 to 3 feet thick, and then 
the Lincolnshire Limestone rests on the Upper Lias Clay.t 
Fossils are nof, as a rule, very abundant, and they are for 
the most part in the condition of surface casts and internal moulds. 
Occasionally, however, they are well preserved, as in a stratum 
at Aldwinkle, where tf the substance of the shell usually remains, 
sometimes even, when first exposed, retaining the nacreous 
lustre."! In this case the rock is calcareous sandstone. 
* Judd, Geol. Rutland, pp. 32, 33, 90, 91, 101. 
f Ibid., pp. 91, 95. 
j Ibid., p. 98. 
