THE MIDDLE LIAS OF NORTHAMPTONSHIRE. 
297 
The sometimes green oolitic character of the rock has 
been described as due to grains of silicate of iron; this may 
be so in some cases, but, so far as my experience goes, it is 
more often due to concretionary particles of carbonate of lime, 
coloured only with the silicate or phosphate of iron. This is 
also the general character of the matrix of bed “ L ” as found 
near Northampton. 
The weathered portions of the ironstone are richest in iron, 
and, therefore, most preferred, chiefly because the carbonate 
of iron has been oxidised, and converted into the brown 
hydrated peroxide by aid of the gases dissolved in percolating 
water, and the calcic carbonate partially removed by the 
same water. Tlie stone in the immediate neighbourhood of 
cracks and fissures, at least in superficial beds, always shows 
this same change of appearance and composition. 
There are some hard stone beds near the base of the 
Upper Lias with intercalated clay beds, which, over much 
of the district marked as Marlstone in the maps of the 
Geological Survey, cover the Rock-bed. These beds, by 
affecting the intake of water, and consequent weathering 
of the Rock-bed, have considerably influenced its character 
as an ironstone or building stone. 
The whole of the Middle Lias, as we have considered it, 
is ferruginous, and some of the thinner hard beds in the 
“ Margaritatus ” Zone are decidedly rich in iron, but because 
of their thinness and variability, they require little con¬ 
sideration. The top bed of the “ Margaritatus ” Zoyie in 
Yorkshire is worked as an ironstone under the name of the 
“Bottom Seam” or “ Avicula Seam,” but we have nothing 
which could be identified with it on that account. 
The thin layers of ironstone found in the “ Spinatus ” 
Zone at Market Harborougli, and in the “ Margaritatus ” Zone 
in some other places, exhibit, in addition to concretions, the 
cellular or box-like characters met with so commonly, and 
on a large scale, in the Northampton sand. The casings are 
highly ferruginous, but they generally contain clay or other 
matter in the interior. They are supposed to have been 
formed by the transfusion of the ferruginous matter from the 
interior outwards. 
Agriculture.— The marlstone yields a rich red soil, well 
adapted to the growth of corn and other cereals, and also 
root crops, and being very porous, it suffers very little from 
wet seasons. It is probable that the richness of these red 
lands is largely due to the phosphates, so often present in the 
Rock-bed. Commonly near the base of this bed flattened 
argillaceous nodules occur in considerable abundance, 
