88 DR. A. H. COX: REPORT ON MAGNETIC DISTURBANCES IN NORTHAMPTONSHIRE 
I. The Geology of the District. 
The geological structure is simple so far as the surface Formations are concerned,* 
The succession among the strata that actually crop out on the surface is as 
follows :— 
Glacial . Boulder clay 
Great Oolite series 
Estuarine beds . . 
Jurassic . Northamptonshire 
iron-ore. 
Thickness. 
Stony clay.Covering up to 50 
feet thick on the 
Upper Lias . 
Limestones and blue clays . 
Vari-coloured sands and 
blue and grey clays with 
limestone bands. 
Oolitic iron-ores, green car¬ 
bonates and red oxides of 
iron, the upper and lower 
portions are ferruginous 
sandstones. 
Blue clays.180 feet. 
high ground. 
Up to 30 feet, 
15-20 feet. 
14-18 feet. 
The distribution of the various beds is shown on the map (Plate l), and the sequence 
and structure are illustrated by the section (fig. l), which is taken along the line of 
the tunnel at present in course of construction by the Ebbw Vale Co.f The strata 
are seen to lie almost horizontally, but there is locally some gentle folding, and 
occasionally some minor faulting. 
S.E. 
N.W. 
Tunnel mouth 
Road to 
Irthlingborough 
lOId workina 
Quarry 
N!2 Pit 
UMilel 1/2 Mile] , it/ 4 Mile 
Upper Lids WMNorthamptonshire Iron-Ore ^MEstuarineBeds ^SGreat Oolite W^Boulder Clay dUMade Ground 
Fig. 1. Section along line of tunnel west of Irthlingborough. 
The Upper Lias clays form a floor to the whole district and are only seen in the 
valleys. The Great Oolite series gives rise to a plateau averaging 270 to 300 feet 
* Geological Survey Map: Sheet 52 (1 inch old series). 
t The section was reduced from a large-scale section kindly supplied by Mr. Thos. Falcon, Manager 
of the Iron Mines. 
