124 
THE LIAS MARLSTONE OF LEICESTERSHIRE. 
map gives a true idea of the area where the Rock-bed may he 
profitably worked for iron. In two distinct ways the Marl- 
stone area shown on the map is very much larger, and in one 
way it is a little smaller, than the productive ironstone area 
met with in the field. In the first place the Marlstone Rock 
is, over a large portion of the district (namely, in the whole of 
the Rutland area, also south of Tilton and north of Caythorpe), 
too thin to work. In other places where it normally possesses 
a workable thickness, the whole or a large part of the upper 
or iron-bearing beds have been removed by denudation so 
that only the lower or unproductive arenaceous beds remain. 
This will be the case more particularly on the hill slopes. 
In some cases, too, the Marlstone may be covered with a 
thicker capping of boulder clay than it would pay to remove. 
For these reasons the Marlstone area is larger than the Iron¬ 
stone area. On the other hand, where the prevailing dip carries 
the Marlstone Rock under newer formations, the ironstone, 
when present, may be followed for a short distance, until the 
“head” becomes too great and the rock gets too calcareous for 
profitable working. In these places, then, the Marlstone area 
delineated on the map may be somewhat smaller than the 
available Ironstone area. It should also be borne in mind 
that in the productive region the ironstone varies considerably 
in quality, its richness increasing apparently in proportion to 
the amount of atmospheric disintegration it has undergone, 
and the greater completeness with which oxidation of the 
iron carbonate and removal of the calcic carbonate have in 
consequence been carried. 
After making all due allowance for the above causes of 
limitation and deterioration we cannot but be impressed with 
the vastness of the stores of iron which must be contained in 
this field. When we remember that the workable ironstone 
averages seven feet in thickness and covers many square miles 
of country, and that every acre yields 2,000 tons of ore per 
foot thick (or a total, say, of 12,000 tons of ore and 4,000 
tons of metallic iron per acre), we see what an immense 
amount of mineral wealth is contained in the Marlstone Rock 
of the Leicestershire district. The proximity of this stone to 
a coalfield, viz., the Notts-Derbysliire, is also greatly in its 
favour, and enables this Lias ironstone to successfully compete 
with the somewhat richer but more distant Northamptonshire 
ore. It is impossible to do more than give a rough estimate 
of the total annual output of the Marlstone ore in the Leicester¬ 
shire district. No complete mineral statistics are published 
for this field, and if there were they would not be reliable, 
seeing that new workings are constantly being opened out. 
