AND LEICESTERSHmE AND THEIR RELATIONS TO GEOLOGICAL STRUCTURE. 1J7 
a thickness of 120 feet. Throughout this thickness there is no ferruginous bed 
which could cause any magnetic disturbance. Above the Upper Lias there is at one 
locality, around Waltham, a thin representative of the Northamptonshire Iron-ore, 
covered by a few feet of sands and Inferior Oolite Limestone. The Northamptonshire 
Iron-ore round Waltham is limited to a patch of country measuring about 2^ miles in 
each direction. Judging by the effect of the same rock at Irthlingborough, where 
oxidised at the surface (see p. 89), the few feet of ironstone in this small patch of 
country cannot give rise to the large magnetic disturbances noticeable at Lempstone, 
14 miles away to the west. 
(b) The Marlstone Iron-ores. 
The Marlstone Ironstone of the Middle Lias represents an altered oolitic limestone 
in which the original calcium carbonate has been replaced by ferrous carbonate which 
was subsequently oxidised to the hydrated ferric oxide limonite. The bed is 
extensively worked along many parts of the outcrop, and in some localities has been 
entirely removed. The iron-ore itself is from 7 to 14 feet thick, and it rests upon 
15 to 20 feet of ferruginous limestones and calcareous sandstones—the so-called 
“ Sandrock.” The strata are practically horizontal and they stand out from the 
softer clays below, giving rise to a ridge of high ground. This ridge sweeps round 
the town of Melton Mowbray in a crescent, breached at one or two points by valleys. 
The inner diameter of the crescent is about six miles. Despite the wide outcrop of 
these iron-ores, they cannot be held responsible for the magnetic disturbances, for 
reasons to be given later (p. IIO). 
(c) Formations below the Marlstone Iron-ore and above the Coal Measures. 
Below the Marlstone iron-ore with its accompanying ferruginous sandstones lie the 
Middle Lias Clays, measuring from 100 to 120 feet thick.. Below them comes the 
Lower Lias, composed of blue clays with thin limestone bands, the division having 
a total thickness of 670 feet. Throughout the clays there is, in the district now 
under consideration, no important ferruginous band. Only in the north-east of the 
district does there appear a band of ferruginous limestone, seldom exceeding a foot in 
thickness, which is the local representative of the important Lincolnshire iron-ores. 
Below the Lower Lias are the Bheetic beds, 30 to 40 feet of shales with thin lime¬ 
stones, and below them the Keuper Marl, red and green marls with a few bands of 
gypsum and numerous bands of calcareous sandstone or “ skerry.” This Formation 
attains here a thickness of about 630 feet. 
Below the Keuper Marl in the area around Nottingham, is a considerable thickness 
of sandstones, namely the Waterstones and the Bunter Beds, the two sandstone 
formations together attaining a maximum thickness of 500 feet. The sandstones are 
VOL. CGXIX.—A. 
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